I can't help you with value of the car you have a line on, but I do try to track the classic brit car market and I have to say I don't see twin cams come up for sale very often at all. The other hot rodded 4s, Healey 100Ms and TR3Bs, seem to come up fairly regulalrly, but I never see Twin Cams on Ebay or bring a trailer, or BCF.
I do think the things are undervalued, the really good factory 100Ms are bringing over 100k, although the Twin Cam had teething problems, the specification and upgrade compared to standard is much bigger and more exotic on the twin cam, don't remember the exact numbers, but production numbers not that different, the MGA has a much better chassis (this from a former 100 owner who loves them, but have driven MGAs, they feel 10 year or so more advanced).
Anyway, know we buy them cause we love them, but a resto twin cam should be a good investement as well!