dvla dont need to know about any mods in the uk they are just interested in who owns the car. Tell the insurance people what the bhp is. if its 400 then its 400. i alwasy tell them the higher number.
cant realy help on the market value of the car unfortunately, and it would depend from one car to another. And its different over there. id say add the value of the parts, half it, then add it to the value of bying a stock supra?
Just a wild guess. Or look for a similar car for sale.
for example stockk tt (a nice one) £8000 - parts for single conversion with decent parts (£12000), other misc mods, seats, bodywork £3000
so. £15000/2=£7500
£7500+£8000 = £15500
you can easily get a single for that
http://www.mkivsupra.net/vbb/showthread.php?294428-Single-6-speed-facelift-500BHP-%A310-000
http://www.mkivsupra.net/vbb/showthread.php?297520-single-turbo-aerotop
http://www.mkivsupra.net/vbb/showthread.php?277223-UK-6-Speed-Single-%28converted-from-auto%29