Low budget sleeper you say?
Buick Regal GS (got my 2002 for $10K 2 years ago, Earlier model years back to 97 with higher mileage can be had for a pittance and the 3800 motor with the eaton blower is as rock solid a base as you can hope for.
Buick exterior, supercharged 3800 under the hood. Replace the grandma suspension with swaybars, trailing arms and strut tower braces from a police package impala ($150). Upgrade the front brakes from the same (some guys even do dual piston setups off an f-body) ($250-400). Next, do in intake ($200), slap on some duals off a pontiac GTP ($150ish, cheaper if you find a GTP guy throwing away his stock exhaust for something loud), downpipe and ported manifolds or headers ($350-500), reprogrammed PCM ($100), drop the pulley size ($60).
You want to go more extreme? Short stack intercooler and a smaller pulley ($600) gets you into the 12's, but now we start getting expensive because you'll start needing to harden your transmission. Even intercooled turbo setups are out there ($3000-4000) and there are several running 11's with 10's not out of the question without looking like a race car (gtp guys have done it, no Buick's that i know of yet)
It's like strapping a ****ing rocket to you living room couch. Leaves mustang owners and riceboys looking confused and emasculated.