Coolant seal failure
 

If the FD has a coolant seal failure, what all needs to be replaced and on average how much would it cost? With the failure, does that mean the engine is ready to give out too?

Coolant seal failure means you need a rebuild. Call for estimates from a reputable rotary shop in your area. Go to your local forum to find out who does good work.

https://www.rx7club.com/south-rx-7-forum-34/

It means the engine *has* given out.

I purchased my car in this condition. You will need a new motor, or a rebuild.

-Geoff

and obviously don't just let the car sit with coolant in the engine or it will be more expensive to replace the now damaged rusty parts. do a search on how to store the car with a bad water seal until it gets the repairs it needs.

Coolant seal failure is the rough equivalent of a head gasket. Short term there isn't alot of danger to the rest of the motor, but if you let it go too long you risk errosion of the coolant seal grooves. If that happens you'll be buying an iron.
Sealant failure alone requires complete disassembly of the motor. If you have a shop fix that, there would be little additional labor costs to replace the hard seals at the same time. Side seal clearancing would be the only obvious exception I can think of off-hand.
This is assuming things like housings, irons and bearings are all re-usable.That's why everyone just considers it time for a rebuild.