Don't cheap out on stuff. Q45 diff is more of a pita than its worth in your car. Just because someone gives you heroine and a needle doesn't mean you should start shooting it up your veins. They also eat up more power than the R200's. Don't be another local 240 lobster. Buy a decent diff or atleast a vlsd *they are NOT that expensive - I've seen kaaz 2way diff's for s13s go for like 700-750 bucks new vlsd's can be had for cheaper.

I don't know why you are worried about the brakes. You probably aren't breaking over 115-120whp (if you are lucky). You aren't going to be generating the speeds needed to justify an allout upgrade. Just get decent pads, braided lines - replace fluid and call it a day.

Coilovers on the cheap (megan racing even though I am not very fond of them). Also get subframe spacers, tension rods, tein tie rods, strut tower bars and fender braces maybe (all stuff you don't have to spend an arm and a leg on).

I wouldn't spend any more money than that if I was you until you get some driver mods. Once your totally comfortable ringing your car out at that level than start worrying more about power (i.e. engine swap brakes e.t.c.).

You aren't really going to start having alot of fun until you dump about 5-7k in your car. Recommend just getting a job and saving up more than anything.