I found other 93 rx-7 that the owner told me it will need a rotor and housing and a seal kit, my question is how much can cost to fix that car?

Or if it's difficult to fix that, because the price of the car it's a good deal.

 

That would be an engine rebuild, and it can run a few thousand dollars if you're paying someone to do it.

 

How much is the guy selling the FD for? List its condition and parts/upgrades it has.

 

It would be cheaper to fix the wiring harness in your car. Especially if you take it somewhere other than the dealer.

An engine rebuild will cost you a minimum of $5k and you'll probably need to replace the engine harness as well since if the car is that cheap, it probably hasn't been maintained.