Answers to your questions;
1. Rims; 16 inch will probably fit with little risk of wheel/body interference. You could fit 17 Inch'ers but then you will probably need to roll the arches, and not lower it more than 30 mm.
I know this isnt a sportwagon, but as you can see the 17 inch alloys is a snug fit in the wheelarches.
There are many different manufacturers, all of them are basically good, avoid rims made in turkey though.
2: Red koni's are nice, but why not go for the yellow ones, they are adjustable.
3: Regarding the Turbocharger, I fitted one to a series 3 16v. Although it ran great, it took a whole month to fabricate a special exhaust manifold and exhaust pipes. Plus making up piping between aircleaner, airflow meter, turbo, intercooler and intake plenum, was also a pain in the ar*e. Not to mention oilpressure piping, oilreturn to sump, Oilcooler and coolant supply to and from turbocharger. It ran with a moderate boost of 0.4 bar, but if you plan to take it higher than that, new forged pistons should be fitted too. And in the end, I crashed the engine, in a courageus attempt to outdrag a pug 106 GTI.
In short; dont do it. Fitting a Supercharger of the roots type is easier (cheaper), but the charger itself cost a few quid more than a turbo.
4: Exhaust, dunno, personally I find Sebring exhaust a good compromise between price, sound and durability. And they fit the cutout in the rear bumper. Some aftermarket systems dont, and that looks downright silly.
5: Filters, I know I'm gonna take a beatin/slaggin' for this. Dont bother with induction kits, they are noisy, and I do not believe they produce more power than a standard filter. However, buying a KN insert could be a good idea, as the filter can be cleaned and reused. But if you plan to fit a supercharger or turbo you have to make up your own airfilter solution, and that might turn out to be a Cone type filter. What I'm saying, dont buy a filter untill U have decided on the forced induction matter.
6. Brakes; Dont bother with disc's in the rear. They require more service, and since they only do, say 20 percent of the braking, then keeping them in good condition will suffice. ATE powerdiscs are renowned for their stability towards runout, and Mintex pads are very heat stable (no fading), yet fine for everyday use too.
Hope this helps. Since I live in Denmark I have no idea what stuff like that cost in the UK.