unless it's a 109 you'll really struggle to put a disco gearbox in it as the bell housing is about 8" too long and the gearstick is mounted way back on the gearbox, you'll need to carry a passenger in the rear tub at all times to change gear for you (not to mention the new crossmembers you'd have to fit)
chopped up disco chassis under it and to remain legal you'll be in SVA (or whatever it is now called) realms, Q plated and paying tax
i run both my old Series with just the disco engines and diffs which means no identity quandary thus keeping me well within the points system
if it's a Series 2/2a and there is any difficulty with retaining it's original number plate it "might" be worth joining the Series2 club as they are officially recognised by the DVLA as the experts in proving the age of 2/2a's (depending on how many bits have been replaced over the years), keep any old tax discs and mots if you can to assist along with the logbook, DO NOT start any work on it before it's been inspected by a club "expert" and signed off (they will NOT sign off anything that's NOT kosher), this doesn't automatically mean that you will be able to retain the original plate but it gives you the best chance
value generally starts at scrap value 250-300, but depending on condition/type/level of fitment (make a good assessment of rot/problems) it could be worth 500-600 without too much trouble, just look at ebay and see how silly some final prices can be for projects, they're getting stupid on there