the block your thinking of is a closed deck, found in early cars usually the ra model.
I would sy condition and how it's been looked after is more important than mileage. My car has around 110k on it, no history but is in great condition and still goes well, pulling strong.
As for buying one, Coulty's on here is an absolute bargain and although its modded more than you want i'm sure after running one for a while you'll want more so why not buy one already finished?
Johnny