I went through all this a while back, half decent Gti at stage 2+ still as solid as a rock with 100K on the clock. A decent BYD came up for a steel so that's going in. Will have cost me £2,500 all in when fitted and I'll be around 350 ponies or so with existing hardware on.
Oh forgot the £1K on LSD as well
A new car is a big outlay and the mk5 with a few styling mods can still look superb. If your car is in good condition still then stick a CDL in for not much outlay and run it for a couple of years.
When you sell up I even think a low mileage CDL conversion might attract the enthusiast more so in a couple of years rather than something like a 100K AXX or BWA in the engine bay, plus you'll have had a couple of years of
Also a great base for a track weapon.