I just purchased a 1990 Gt4 myself - beside one little run of bad luck with a drivshaft the car is an absolute dream to drive .. its very comfy - everythings easy to reach - they are easy to drive and boost IS your friend
and as for sluggish - HELL NO - mines not overly modified but it hammers and handles very good but for me that came from a suspension modified ta22 - i prefer a slightly firmer ride than stock ( easy fixed with springs, rear swaybar and bushes )
The only drivetrain IF is the Clutch - they really need a heavier duty one to counter the boost - so if this is in - big bonus - if not - its definantly worth budgeting for this to help with your cars life.
I payed 11k for mine - its very clean - very fast for a non turbo experienced person like myself - and has So much potential for " go harder faster and smoother " im already writing the imaginary cheques !
I say if you want one - GO FOR IT ! they are cheap compared to a rex ( try 25K for the entry point for a rex thats probably had its guts thrashed out by some young dickhead ) compared to 11 for mine - and its neat tight and reliable .
If you search enough you will find good ones out there - just check into the background of tyhe driver with a few logical questions ( ie if you can see its been made fully sic - he probably flogs it to hell ) whereas the middle aged owner would have probaly bought it as a weekend fun toy - and had fun but not thrashed it ..
BEWARE the JDM one or the " slimline " model - as parts would be even harder to get than the aussie delivered widebody ..
they share the same body as the standard st184 fwd celica - but have Gt4 running gear - though im not 100% sure if they are as stromg in components as the ones we had delivered out here.
and with the 10K you can save from not buying a rex - you could tweak the suspension , buy a good stereo - get a good exhaust setup thru it , a boost controller and turbo timer - AND afford the Insurance ( 2200 for rating one with shannons - full cover ) and have something thats a dream to own and drive