Tom:
I also have a Robbe U-47 and I am thinking of modifiying it to static or "semistatic" diving (this is, the possibility of changing trim from scale surface to fully flooded with water line over the deck).
I have already tryed the second option and I think is the most economic eficient procedure: cheap, easy and most regarding.
If I decide for full static diving I would also like the 750 cc Engel tank, but the modification to my existing model should be quite large. And what about the cost?, as I think the best power solution should be Li-Po cells and sure they are not cheap.
Regarding electronics, I am not an expert, but have some experience and I may say you that leveling should only be dynamic and what you need is more or less what helicopter guys call a "heading lock" gyro.
This is basically a piezo gyro sensor with an integrator calculator that taking account of g force value, sense as well as time, is able of calculating the drift from the original position, so it may take just precise corrective action.
I have tryied one of this gyros on my series dynamic diving submarine without much success, rather I prefered it in normal gyro mode rather than "heading lock". But it is mainly due I conected it to the rear dive planes servo and when you send some command this type of gyro produces control surface full deflection.

Jesus Cardin