I'll chime in here on the 7M / JZ subject.
I own both - an 89 Targa 2J single swap and an 89 Hardtop 7M. I like the Targa better, so it's the one I've been driving. Kinda miss the aircon that's in the 7M car, especially in the winter when the windows want to fog up.
Personally, I'm of the opinion that anyone who is going to upgrade past a certain point might as well go JZ. If you are planning on upgrading the ECU to a standalone, moving to a bigger single turbo, bigger IC / hardpipes, and fuel to support all this, then it's entirely an aesthetic choice what engine to put in - and there's no debating the fact that the JZ engines look miles better. The cost difference between building the 7M and a JZ to this level isn't worth considering, it's a fraction of a percent at most.
Also, the aftermarket support for the JZ engines is far better - try to find a cast exhaust manifold for the 7M, for example, and you're digging through eBay Japan, hoping to find a used Greddy or HKS unit. If you need one for a JZ engine, they are available off the shelf from most of the big Supra shops stateside.
If you're not going that far, then keeping the 7M is another very viable option.
PS: It's still very possible to build and run the monster 7M - and in some ways, it's easier, because you don't have a lot of parts to pick and choose from.
