Upgrading a 1991 Mazda RX7, help?

[Joey]

I'm buying a project car and it's a little... well really bad off. I would like to install aftermarket parts as opposed to OEM so I don't waste money on going OEM first then buying upgrade parts later. Do you guys know what would be best? I'm having trouble finding stuff for a '91, and I know it needs a new exhaust and the engine needs a bit of work, so I've found a Borla cat back system, but do I need anything else for the exhaust? any help welcome