What should I look for

I thought that they were all the same in-line 6 cylinder with a turbocharger. Is there a difference?

The stock engine in the Mk 3 Supra is the 7MGE (7MGTE in turbocharged models which wasn't introduced until the 87 model year). They tend to blow head gaskets because the head bolts weren't torqued down properly from the factory. The factory service manual specifies 50 lb-ft of torque for head bolts when the proper number is actually 70 lb-ft. That's the only real problem that I can think of, although there may have been some electrical problems. Honestly, you're going to spend a lot of money to add any power to the 7MGE and even the 7MGTE is going to start out more powerful but end up hitting around the same wall. The motor that makes crazy power on stock internals is the 2JZ-GTE which was the stock, turbocharged engine in the Mk4 Supra, from 93-98 in America (up until 02 in Japan). If you want to make power, you're better off swapping in an LSX or 2JZ than building the 7MGE. But with time and money, the 7M can make in the region of 700 hp.