first of all your best mod is going to be a daily driver...buy a 800 dollar civic or corolla or something to get you around in case your working on your car,break something, or just dont want to drive around in a stiff, loud, hot etc. car which gets attention from police all the time..Trust me.
as far as mods go i say if your just getting into drifting a 1000+ coilover kit is not going to make you any better than some used good condition megans, isis, d2 etc etc ....there are plenty of semi-pros and some pros using megan, isis, and BC. I personnally love my Stance gr + pro but...... Skill>mods.
if you do really well and outgrow your coilovers you can always upgrade latter.
id say stay away from ebay coilover sleeves and probly tein basics. everything else would be good enough for learning.
as far as a diff id say weld it but you might get tired of it therefore you should get another car for a daily.(as previously stated). if its your only car and you dont want to weld it then just get a j30 or keep the open and swap in the welded for events...with practice you can swap them in about 45 minutes.
on an s chassis id also suggest getting some inner and outer tie rods w/ extended spacers...stock angle is sooo limited. and make sure you do maintenance to you car as well as mods...i know too many people who mod up their cars while they have leaks,worn bushings overheating etc. but think they're cool cause they bought an exhaust.
there is a reality that if you are going to drift your probly going to crash or break something so consider if this is a sport you want to pursue with your new car.
learn your car, drive it, go to events...the rest will follow. Good luck