www.cetmerifles.com/forum/index.php?sid=57217a905e9aa35cff37aec3759a54c3 cetmes are hit or miss as far as operation goes. there are a number of common problems with them, or you may be lucky and she runs like a charm.
most of the problems aren't that hard to overcome, and a visit to the cetme forum, linked above, will help you along. read their faq section.
i paid way too much for mine. it's a fairly early one with a cast stainless reciever, and it too had some issues. it had a canted front sight that the century wondersmiths made alright, and a barely passing bolt gap that i made better by going to larger rollers.
mine runs awesome, but their are plenty that just piss off their owners by being unreliable.
for $250 though, why not? if it's your friends, you get to take for a test drive first to see how she works.
and on the hk parts interchangeability...a bunch, although not all, of hk parts will fit right on it.