So the first thing to think about and research with mobility chairs is batteries. So far every one I've seen runs off 2 12v Lead-Acids in series to make a 24v pack (nominal voltage). The Lead Acids should be Deep-Cycle AGM type, not car batteries. The reason for this is that car batteries are meant for what is called "cranking amps"- mainly a very high current discharge rate for starting your engine. Then the alternator charges up the battery cells with motion caused from tire rotation powered by gasoline.
Our situation is different. We need long-lasting batteries that can discharge high current, but not as high as cranking amps ( ~35A will be max on my scooter). They also need to have a long life-span and support deep discharges for those days when you are traveling a lot or, like me, buying groceries. I recommend going gun-ho on batteries...getting the highest capicity that will fit in your chair/scooter. You will thank yourself later.
Battery capacity is measured in amps (Ah). I recommend a google search to get the basics down on that. Amps (x) voltage = Wattage which is your tota power. So the higher the amps the farther (and often, faster) your chair will go! Speed wont be directly proportional, but the less heavy drain there is the higher the voltage will be.