The challenges, particularly when they involve 2nd hand cars, are primarily entertainment, rather than factual. They are obviously scripted. Neither Clarkson nor May were really in their hot hatches when they were blown up. Hammond didn't really roll his Nova twice. These types of challenges are not meant to be consumer advice pieces.
Yes, they could've done a proper review of late 80's/early 90's hot hatches, telling people what to buy and what not to buy, finding the best on the market, road testing and comparing with modern equivalents, but that would've been a little dull. That's why this sort of stuff was quickly dropped (along with Jason Dawes) after the 1st series of 'new Top Gear'.
Leave that sort of stuff to Fifth Gear on Discovery (although I suspect they might also go a bit more 'entertainment' to survive) and let the boys continue to cock around.
'New' Top Gear, is primarily entertainment and aspiration. Very little is dedicated to genuine consumer advice any more. And what little there is, is done in an entertaining way, rather than journalistically. When was the last time you saw a new base model (the sort of car most of us end up buying) seriously reviewed on Top Gear?