After some further development I landed at the current concept:
What I use is:
Board: Naish Hover 160 with 20l
RC: AlienPowersystems (for electric skateboards) and a waterproof document bag
Prop: Yamaha 7 1/2 x7 (biggers are more efficient than smaller props)
ESC: Multiplex Roxxy Blcontrol 9100-12 Opto modified with Peltier element attached to a aluminium plate on top of the Box (better heat shift and circulation within box)
Accu: Turnighy high capacity ( but I want to shift to 18650 Li Ion types by LG)
Profile shaft and tube: custom shaft and welded on tube(Da45mm) with aluminium front cap
Bearings and sealings seat: aluminium milled
Motor and Gearbox: Inrunner motor special gearbox, output 100KV
50 min of flight time is no problem with 36Volts and 16Ah. Max Speed ~35km/h. I started with 8S, this is also possible but speed was only 25 km/h.
Next step is -of course- 12S
Starting power consumption is ~1,8kW, flying speed at 25-30km/h is 400-500W(!) I was very surprised with that! A friend of mine with 85 kg needs 30% more power.
The concept is quite cheap, parts cost ~650€ excluding board (350€ in summer sale)