SR20 in a ke70/ae71 would be insane in terms of power to weight, but would prob rule it out of most racing classes.
what class do you want to enter?
if it were me with 15 grand and a 71 i'd just chuck a 16v 4AG in, twin side draught carbs, big cams, front coilovers (8kilo rates) and rear springs n shocks (6kilo rate), small brake upgrade (JDM AE86 or T18 disc/caliper), rose jointed LCAs, sway bars n adj panhard rod, T18 diff with a mech 2way, neg offset 15 x 7s with semi slicks, fixed back bucket seat, some sort of sports wheel and a few extra guages to keep an eye on engine vitals
that should easily come in under 15 grand, and you'd have an immensely enjoyable car you could race or slide, and it'd be pretty reliable too.