having owned a 350gt coupe and now on 250gt sedan
1st question - pretty much, they are all rwd and all good just power is low on 250 and 300 compared to the 350
2nd -a few parts are compatible yeah, id guess the exterior would be no difference in the skyline models, but mechanically they have diff engines obviously but some parts there are even ok to pick and choose from the maxima etc (depending what you want)
3rd - 250gt - I think about sub 8 seconds? - someone correct me there if im wrong, it does feel like it though. and fuel consumption is good, there's a thread already started here somewhere if you use the search button to see what people have posted on the fuel consumption records etc on all variants - but for me amazingly I got 700 kms out of a tank one time in the 350gt (have no idea how) and iv made it to 600-650 mark a few times too, but usually in the 500-550 mark, with the 250gt about the same 500-550 mark too.
other - suspension obviously different for the coupe and sedan, the sedan is way more forgiving and soft and nice ride - would be awesome for interstate trips, power is ok and enough for day to day driving and overtaking. make sure you get a series two if its a sedan - they look much better (with the face lift and back tail lights a bit different from s1) and also the interior is much improved than the s1 also less window motor problems and electrical problems all together are lowered than that of a s1