I have had my supra about 8 months now, and its on 130k miles and about 13 years old. The engine dosn't miss a beat and i've never had any issues with it. The rest of the car is showing its age though. In the past two months I've had my rear wheel bearing and hub changed and a rear top arm replaced.
Don't be put of though, because I had 6 months of good motoring and I'm sure it will be a while till i have any more problems.
If you got money to replace the engine and get a turbo kit, could you not put the extra money towards a low milage newer supra?