Several options to think about....
Option 1. If you build one from scratch what ID would you use? As to stick within the law and the rules of the DVLA you need to retain at least 8 points, details of this can be found on the Internet if you search for "inf26.pdf"
Option 2.
Choose in your mind which model and engine type you would most prefer and go and buy the very best example you can possibly afford.
Option 3.
Again once you decide on model and engine type, buy a scruffy donor vehicle and have it rebuilt...
So base vehicle price plus the following...
Rebuild costs can vary wildly however, for a galv chassis, bushes, shocks and springs will be approx £3k-£4k inc parts and labour depending on specification.
A full build including galv chassis, galv bulkhead, galv doors, full paint, rebuilt engine, gearbox and axles can range between £15k to £25k again including parts and labour but depending on specification.
So if you consider you can spend upto £30k on a shiny new TDCI and in the next 10 years there is a chance it will fall into the rotten chassis brigade, a rebuild should last you a damn sight longer for a lot less money.