The best shower for an off the mains system?

You say that the pressure varies? Do you not have a cold water storage tank? From my experience if you don't have mains pressure electric showers can over heat and cut out, this could be the case if you install one. If you have a cold water storage tank you can install a booster pump, giving you better cold water pressure: Grundfos UPA 15-90 Domestic Cold Water Boosting Pump (Stainless Steel Version) for example. this would also give you better pressure at the taps!