The build.

Quick answers to your brain-dump

• Sideways double bed at 165 cm? Yes. Any L3/L4 Sprinter gives ~1.65–1.70 m clear width after panelling — you fit east-west, flat, no flares or recesses needed. Build the bed low (near wheel-arch height) to keep the full width. On the XLWB (L4) a sideways bed frees the whole rear as a garage — ideal for your tools + e-bike.
• Electric instant shower heat? No — physics says ~6 kW. Heat water with a diesel water heater (Truma Combi also does cabin heat), engine heat-exchanger as a free bonus on driving days. Everything else electric.
• Your 12 kWh battery is well-sized (modest day ~3.6 kWh, hot gaming day ~10 kWh). Pair with a 5 kW / 48 V inverter, ~600–800 W solar, and alternator charging as co-primary.
• All-electric / no gas works — induction + microwave + kettle are fine on energy, but they drive inverter size (run one big load at a time). Going gas-free removes a vented locker + certification.
• Solo + occasional guest: a pull-out single→double or a powered lift bed (rises by day) both work; passenger swivel seat = your desk + workstation.

Sizing recommendation

• Battery: ~12–15 kWh usable (2–3 days at modest use) — your existing 12 kWh is bang-on; going bigger adds weight/cost for little gain.
• Inverter: 5 kW / 48 V (covers kettle + induction + PC together; 48 V keeps cables sane). Prefer 48 V architecture — it's what makes A/C + induction + microwave co-exist.
• Solar: fit the max roof (~600–800 W ≈ 2–3 kWh on a good summer day) but treat alternator/DC-DC charging while driving as co-primary — no roof array closes a 10 kWh winter day, so you'll drive or shore-charge.
• All-in-one vs DIY: a Bluetti Apex 300 (2.8 kWh, 3.8 kW) only works expanded (base + 3–4 packs ≈ 11–14 kWh); EcoFlow Delta Pro Ultra (7.2 kW, 6 kWh/battery) is the strongest turnkey but heavy/pricey; a DIY 48 V 12–15 kWh bank is the best Wh-per-$ and Wh-per-kg. The shipping trade-off is real: a removable station ships lithium-free easily; the DIY 12 kWh is best for use but a fixed install (or you ship lithium-free + rebuy in EU).

Eberspächer Zeliox Neo 4000 — clever box, wrong fit for you

• What it is: a sealed 48 V all-in-one — LiFePO4 battery + 4 kW inverter (8 kW peak) + solar MPPT + alternator DC-DC + mains charger + 12 V output in one 43 kg box, with a touchscreen, 4G app and 5-yr warranty, from Eberspächer (the diesel-heater brand). Base ≈ £3,100–3,720 (~A$6,400–7,600).
• ❌ Caps at 9.6 kWh (base 2.4 kWh + max 3 add-ons) — below your 12–15 kWh target, no headroom, and dear per kWh (~$1,200–1,400/kWh). Add-on battery pricing isn't even published — you'd have to phone a distributor.
• ❌ Sealed & non-serviceable — the manual literally says "repair is not permitted." One fault kills battery + inverter + all charging at once, and it's an EU-only whole-unit RMA. Bad trait for a van you live in full-time and ship round-trip.
• ❌ Can't use your 12 kWh bank — buying it means abandoning hardware you already own (~$3–5k wasted).
• ✅ Who it's actually for: someone starting from zero, ≤9.6 kWh needs, who prizes one clean plug-and-play box over repairability and isn't shipping internationally. Not you.

• Fixed system, lithium aboard (DIY Victron, or an installed all-in-one). Clean only if the van fits a container (a lower / standard-roof build). For a tall van forced onto RoRo, an installed battery is increasingly refused, or pushes you to flat-rack (+$3–7k/crossing).
• Build light → ship lithium-free → buy the all-in-one in the EU (your plan). For a tall RoRo van this is the cleanest booking, widest carrier choice, no DG premium, warranty starts the day you start driving (serviced in-region), and your house batteries stay put.

✅ Revised recommendation — your instinct holds up

For the tall luxury Sprinter you're leaning toward, build electrical-light, ship lithium-free, and buy an all-in-one in Europe. The van can't container anyway, so this is the cleanest shipping path; it's cost-competitive-to-cheaper once the house-bank replacement and per-crossing lithium premium are counted; and the warranty starts the day you start living in it, serviced where you're travelling. Your house batteries stay home.

Best picks:

• EcoFlow Power Kit 15 kWh (~$22.4k) — the only genuinely van-integrated option (built-in 12 V fused panel, alternator, solar, inverter). ~Double the price and the inverter's 3.6 kW (manage big loads one at a time), but the least DIY, and it ships fine on later legs as an installed system.
• 2× Bluetti Apex 300 + B500K (~$11–13k) — true 5 kW+ paralleled, cheapest route, but you do the most integration.
• EcoFlow Delta Pro Ultra, 18 kWh (~$14.8k) — the only one to clear 5 kW (6.9 kW) from a single box; portable, so the easiest first shipment.

If you instead chose a lower / container-able van (or a pop-top), keeping your own lithium + a DIY Victron comes back on the table — a container ships an installed battery relatively easily. Van height is the upstream decision.

The two honest caveats + what to check

• "Plug-and-play" is ~80%. The portable stations are AC boxes — you still build a 12 V system (fridge/pump/lights/fan), wire solar in, and add an alternator charger (the DPU has no native alternator input). You're deferring the battery + inverter + chargers, not the loom.
• The later-chapter trap. A loose power station legally can't ship inside the van as cargo. So Europe→USA→South America means either re-shipping lithium-free and moving the station separately each time, or hard-installing it (which recreates the lithium-aboard problem). Decide multi-continent up front: yes → lean to the installed Power Kit; Europe-first → a portable Apex/DPU is fine.
• Before buying: confirm EU stock (the DPU battery is pre-order ~Jul 2026; Power Kits sell out), factor VAT (~20%, already in these prices) and export formalities for non-EU legs, and get written RoRo quotes (lithium-free vs aboard) for your van's ~35 m³ from K-Line / Höegh agents.

The decisions that control the build

• Power voltage — 12 V vs 48 V. 48 V is what makes rooftop A/C + induction + microwave + electric hot-water co-exist without melting cables. Choosing it re-prices the whole electrical loom (thicker cable, Class-T fusing, 48 V DC-DC, 48 V A/C). Decide before buying anything electrical.
• Removable all-in-one vs fixed DIY bank. The crux of your "carry-it-out box" idea: removability solves shipping + is dual-use, but the all-in-one big enough for your loads is heavy + pricey; the DIY 12 kWh is cheaper/bigger but fixed. Resolve this with the shipping plan.
• Gas vs induction. A fork, not an add-on: gas → vented locker + bottle + certification; induction → bigger inverter + battery. You lean all-electric — that removes the gas locker entirely.
• Rooftop A/C yes/no. It forces a large battery / 48 V and adds roof height + weight (watch EU 2.6 m car-park/ferry bands). Under-bench is the lower-profile alternative.
• Shower water heating. Diesel (Truma Combi) vs a slow electric tank — settled above, but it dictates your heater choice (don't also buy a standalone diesel heater).
• Bed: fixed east-west vs powered lift. East-west fits you at 165 cm and frees the rear (garage); a lift bed frees the floor by day for an office but needs ceiling clearance + structure. Pick one early — it sets the whole layout.
• GVM plating. Keep it ≤ 4,490 kg to stay car-licence-legal + simplify shipping; a loaded luxury Sprinter gets close, so weight-budget every component (weighbridge milestone in the schedule).