no such thing as 500W or 1000Wmotor, apparently they are the same (probably marked 500W to comply with local regulations). but the controllers are different.
I wouldn't recommend the huge 48V 20Ah batt that you were thinking of. to get an *idea* of the energy capacity multiply voltage times capacity; 48V * 20Ah = 960Wh, and that's is A LOT. for comparison, the most expensive battery that Segway offers is 400Wh.
I'd definitely recommend 48V over 36V, it'll make your rides much more fun. I'd consider 48V 8Ah enough for occasional use, but I'd go with 48V 10 or 12Ah. depending on cell type, a low Ah rating might mean a low power rating too (max amps, given a certain voltage). unless your 8Ah are very good cells, you could have cutoffs and reduced lifetimes due to excessive power demand. with 12Ah, things will go much more smooth with "low" quality cells.
now, regarding range and voltage:
36V * 10Ah = 360Wh
48V * 10Ah = 480Wh
at the same capacity the 48V batt has 33% more energy than the 36V batt. that said, it might be the case that the 48V batt will give you less range.
you see, the 48V will let you go faster; imagine it gets you 35% faster (yes, can be more or less than 33%), but using double the energy per mile that you'd use at 36V. so the 48V batt would take you 1.33 / 2 = 67% the distance that the 36V batt would; but only if you travel full speed. however if you go at comparable speeds, the 48V batt should give you 33% more range.
(controllers become more efficient when they run full throttle, so the last statement is not really exact, but it's close.)
so, with 48V 12Ah you'll have: good acceleration and top speed for fun; good range to travel (similar to a 36V 16Ah, provided that you go at 75% of max speed); reasonable cost and weight. (but if you want great range while going full speed, you might be disappointed.)
