u could make an RB26 gearbox rear wheel but most ppl tend to use RB25 boxes coz they're just as good anyway.. but since u need 6sp then this is pretty much irrelevant..
this guy who lives 5 mins away from me just rebuilt his S15 gearbox, i think total including shipping was ard AUD6k, im not sure how cheap that would be for you but comparing to a HKS sequential, you'll be able to get at least 2 done for the price of one HKS ;)
alternatively, just got for the OS gearset.. ive seen quite a few cars run good times with brutal launches and hard gearchanges with them here, but not S15s tho..
how much power you putting out and what are you using the car for? drags? circuit? if its just for drags maybe go auto ;)