Sounds like we're getting somewhere with a plan, Woody.
Thankfully changing a road car in terms of MOT is a lot easier than trying to make a race car MOTable. So, keeping to the set of rules I think you'd have not problem doing the following:
*Get car back.
*Strip everything out of the interior, sell everything that won't be used seats/doorcards/rooflining etc. Possibly retain 3-point belts for comfort, but they're not absolutely necessary and it will pass an MOT without them. Think R26.R - they've not got 3-points and I know plenty of track cars with MOTs, you just have to go to the right place that understands the regs.
*With funds from above, buy cage/lightweight doorcards/buckets/mounts. RS Sabelts are ideal and quite cheap.
*Rip dash out and send off to be flocked, 'cos it's cool.
*Send rolling shell to Dom Field in Congleton, he's reshelled more Cup racers than you've had hot dinners, he'll have a Matter cage installed in no time.
*Fit lexan windows if budget allows, do away with unnecessary, rear wiper etc.
*Re-assemble car, get it cornerweighted and geo set up properly.
I reckon you could have all that done in 4 weeks if you got your act together and by selling stuff to fund works, it won't hurt you that much financially. You'll have shaved a load of weight out of the car, it'll feel much stiffer and with your current engine it'll go like a stabbed rat. The most important thing will be to use it and enjoy it, there's no fun in it being sat there doing nothing and you living off beans trying to realise a crazy dream.
Get a feel for it over winter with some cheap track days and tuition, then batter it all next season as it is. It'll be an epicly fun car as it stands.
I'd certainly shelve any plans for going F/I for 12 months at least, watch what's happening with the other people doing the hard research for you. It may transpire that 225 Megane conversions turn out to be a doddle and get you the same power as your s/c for thousands less. Patience is key when it comes to stuff like this, let others take the pain and costs.
Oh, and you don't need uprated rear brakes on a FWD car. They do cock all anyway, regardless of size.