As said yellows start leaning of after around 280, though some have run a bit higher but not 300.
I think someone is telling you porkies about the car, there is no logical reason to cam it at this level at all.
At this time of year and that power a 4wd intercooler will be fine, but near the limit, but as said RS500 one MUCH better.
For the bits mentioned above you do NOT need a "fitter", they really are a doddle to do.
I would as said check the basis is ok though first, ie the engine, and change both the fuel filter and the wiring, both easy to do.
As a cheaper option you can get your T3 machined out(compressor housing) to take a large turbo escort trim wheel(T34) with the usual mods, ie 360 bearing step gap seal.
I would give the car a MEGA service everything you can first, as loads been bodged/not maintained unfortunately.
tabetha