Should I change to a manual?

Depends on how badly you want it. The auto transmission should be able to handle quite a bit of power and is pretty decent performance wise. But of course if you really like the feel of driving a manual you could consider doing it. If you do want to do a swap, I wouldnt buy it over craig's list. Instead, consider buying a new or rebuilt engine from a specialist shop, which is really worth it considering the age of the car. Another thing you should take into consideration though is that if you're never going to turbo charge your engine, the 13B will never produce enough power to warrant a manual swap unless you're swapping just for the sake of driving a manual, and even with the manual, there isnt enough power to make it that fun anyway. With the money that you're willing to spend on a swap though, I would recommend just selling your car and buying a rx-7 13B-T or even another similar car that would be just as satisfying. The Z32 TT 300ZX is in a similar price range if you can find one. However, if you prefer to spend less money on the car, I would just have the car checked out at a mechanic and swap some performance parts to breathe some life into the old engine, rather than going for a manual swap.

I have an 82 manual and it doesn't have much power either. If you swap to a manual expecting a noticeable power difference I think you'll be disappointed.

i have an 86 automatic.. i would make it a 5 speed and find a factoryrx-7 turbo.. it will be a beast when you are done

I would not recommend it. Unless you're going to race it all the time, the manual is hell in traffic

Yeah he's right. It depends on where you live I guess. If you're gonna get into major traffic it'll be a pain in the butt if this is your only ride.

I wouldn't go ahead and drop a chevy 350 in it then it doesnt matter if it is a manual or auto...

There shouldn't be any difference between the engines auto or manual... convert the transmission and everything to being manual, and just keep the engine the same.