As the others have stated it all depends on the rules. Minimum weight and if you are allowed to run newer engines in an older chassis.
Chassis
I've been told that the 116 coupe has a lower center of gravity then the 75 but the 75 is lighter when you strip it down. If your minimum weight by rules is above the min. weight of the coupe, I would use the coupe. BUT when you look at the availability of lightweight body panels here in europe I would choose the 75. That might be different in Australia, however.
Lexan side and rear windows and a fully stripped chassis would be the first things to start with. A fully welded roll cage with extensions to the front shocks would be second. You might want to think about seamwelding the body when you have it stripped down for the fitment of the roll cage.
Engine
I would think there's more power to be made with the TS engine, but Jim is more qualified to comment on that
Suspension
I would go for the RS racing suspension for starters. Yes, it's a shitload of money at 1295 euros ex taxes, but it's by far the best suspension if you don't want a full custom set specifically made for your car and driving style. I have it on the car and it's absolutely amazing!
P.S. Barry, reply to my mail also!!![/b]