I've been doing the very same thing for the last month, but I wanted something that was fun to drive just not to turn my licence in to an endagered species, so my list is
*MX-5 $6000-7000 7-8l/100k mark and are reliable as anything, great on fuel and awsome fun to drive. Ultimate drivers car
*Peugout 205 GTi $4000-5000. 8-9l/100k great fun little car, good on fuel but parts are reosonably hefty. Ultimate front wheel drivers car
*E30 BMW $2000-6000 8l/100k depends on the model and if you are willing to ship parts from europe, but maintnence isn't to bad. 100kw 1.8l twin cam, 318is is the go (everyone says BMW's are really bad on maintnence, but the one my friend owned (E36) got a hiding, never missed a beat, couple of little things like the altenator went, and the fuel filter was a touch pricey, but it went really well for 3 years)
*R31 $1000-5000 for hr31 9-11l/100k. cheap, easy to fix, torquey (rb30), wouldn't even worry about a rwd trx, but need a bit of work to be fun and good in the twisties
*R32 $8000-10000 great on fuel off boost 9l/100k, not so good on boost 13-14l/100k, reliable as long as they are in good nick, awsome fun, and would complement the GTR
Couple of not so mainstream ideas for myself were KE20-30 corolla with 4age efi, lock diff and some suspension and brakes, Toyota starlet with suspension and an import 4efte motor(good for 1:10 at wakefield), old school celica's, datsun 1200 coupe, suzuki cappucino (girly but awsomely fun), porsche 944 as a daily
Disclaimer. These are really just underpowered drivers cars and not really practicle Ideas, except for the 31 and the BMW