Enjoy! It's a great car.
Sorry - can't be of much use on the exhaust but on some of the other stuff I can.
I am sure you will have read through the forum but there are a few good threads at the moment. A good thread by MurrayGTV on Rusty Sills/Floor Pan/boot pan/foot wells. Mine is currently in for some work at the moment. Get the sill covers off and wheel arch linings and have a look. Obvious things such as the cambelt w/water pump are a given.
~ Oil Cooler located behind/within O/S Front Bumper underneath. Check the pipes for leaks and also the cooler itself particularly at the elbows. There is an aftermarket Mocal Kit and it is a job that a laymen can do with a bit of planning and effort (Autolusso supply this). A snug fit but it is a significant improvement and you can sleep easy.
~ Brake lines and Rear Brake Compensator (buried above and around the rear section exhaust box). Front Brembo calipers can often need a refurb/replacement
~ Oil filter - has it been changed? It can be a pig to get at and might have been "skipped".
~ Clutch Slave Cylinder - condition check for leaks. Can be seen below and when removing the air intake/resonator up to the throttle body. Easy and cheap fix.
~ On the exhaust - I would go for something more subtle maybe just a rear section. Personal preference. I have a de-cat custom exhaust on my 156 V6 and it is too loud and it takes away from the engine note (fun though). On my GTV I have a Ragazzon rear section only which eliminates one silencer and it burbles but nothing more. I get a lot of pleasure out of hearing the engine mechanism up front.
~ Check the rocker covers around the cambelt end for minor oil leaks.
~ Suspension work - an overhaul transforms the car. The more you replace the better the rewards. Do it yourself and you will get incredible satisfaction from the frustration. Treating the suspension components such as the dog legs and the spring pans (if they don't already have holes in them). Inner rods to rack and track rod ends, new shocks etc. You will have a different car.
~ You will see a lot about GTA Clutch Upgrades and Quaiffe/Q2 Diff upgrades (something I will look into once I get mine back). Again, rumored to transform the car.
~ Lambda's often need changing - rough idling and a garage stench.
~ Air leaks/cracks in the throttle body induction pipe rubber. Autolusso do a Silicone option.
~ Good set of boots won't go a miss.
~ Probably a lot of superficial work also such as door card holes, crap gear knobs, steering wheel wear and tear, paintwork blemishes.
I have found that you can get quite involved with the car even with the most simplistic checks such as spark plug condition, detailing the engine, brake overhaul, gearbox oil change etc. Makes the car very rewarding (when it is on the road in my case). I hate to think what I have spent on mine considering it doesn't have an MOT at the moment but mine was almost certainly in the wrong hands beforehand! I am sure others will be along to offer more doom and gloom but in reality, they're getting on a bit now so you just have to accept it.