"A good few years", but you'll have spent £5k on this before you know it - probably £6-7k in total on all the sheds you've had over the last year. That's 5% down on a mortgage.
This car, with its mileage, will not be worth what a 'clean' lesser mileage Touring would be worth (which isn't a lot all told anyway) even if you do sort out all the issues with it. If you stopped shoveling money into it you could save for one that isn;t in such dire straits in the first place, and is more likely to hold its value.
I'm all for project cars, all for having ones dream cars - especially lower budget ones - as my dream car list is predominately clapped out Jap sheds from the late 80s early 90s, almost all of which can be had for <£4k, and I have no problems investing into such cars and taking losses on them, but there has to be a cut off point. I love the passion you have for it though, that's excellent.