Excellent rifles. The build quality on them is great. I have one, an Egyptian in 8mm, and I love it.
The firing pin issue previously identified is a caution with the Egyptians, as is care in ammo selection/reloading. The general view is that the rifle does not like soft primers and there is a risk of an out-of-battery incident that leads to damage and the risk of injury. Too much firing pin protrusion is also suspected as playing a role in the OOB incidents. Milsurp ammo is good. Avoid the Turkish 8mm as its powder burn rate puts it out of step with the gas cycling in the semi-auto. Mine loves the Romanian from the 70s, and others I believe have had good success with the Yugo ammo. If you're reloading, the suggestion is to use CCI 34 primers (though I haven't seen any of those for a loonnng time) or CCI 200/250 primers.
There is a book, the FN-49 "bible" if you will, that is well worth buying:
http://www.fn-browning.com/fn-49_the_last_elegant_old_world_military_rifle_book.htm. The author, Wayne Johnson, shows up on several other boards as "Goose52" (try gunboards) and is very helpful.
As for which ones to avoid. Aside from the issues identified above, they're all good. The Egyptians, in 8mm, are the most common, generally have been harder used, and the most affordable. Other contracts had the rifle chambered in 30-06, 7mm and .308. Those variations, in a bit shorter supply and often in better condition, tend to draw a little more money.
Prices? $500 seems to be the floor about now, for a "shooter" in 8mm, and not many show up at that price. They go up from there. On the Egyptian 8mm rifles, watch out for those that were assembled by Century Arms from spare sparts. Not because there are any mechanical issues, but they are not "as issued" military surplus rifles.
Good luck in the search. Looking forward to seeing your FN-49 when you get it.