Hello,
1) Depending on which flavour of MOSFET/FCU you buy, it does much more than simply prolonging the trigger contacts life. Most notable features are cycle detection and active brake, they make sure the gearbox stops its cycle in the same spot every time and mitigates lockups; More "high end" units will have precocking, binary fire, adjustable levels on both AB/Precocking, adjustable trigger sensitivity, but these functions are mostly for hardcore CQB use
2) Generally, yes, except for ARES guns, they're shit and their EFCS is equally as bad, avoid. Depends on where you play though, if you only play at woodland sites in full auto, an FCU isn't strictly necessary but still nice to have, even if you're a beginner;
3) Definitely, avoid shady stuff like KWA, Ares, weird shaped guns with non standard gearboxes (Cyma SVD and VFC MP7 come to mind). Every gun will break at some point, tolerances and quality of parts matter but it's still a question of "when", not "if";
4) "upgraded out of the box" doesn't really make any sense. Parts that the manufacturer installs in the factory, even if fancy, don't count as "upgrades". Also most "upgrades" are shit anyway because bad quality - bad installation - bad QC so you may end up replacing them anyway. To answer your specific question, a VFC based Cyma AK will be an inexpensive gun that you can upgrade (for real) later on. I'd also stay clear of anything with a barrel length greater than 10/12" since this parameter matters sod all in airsoft, in some cases a longer inner barrel gives nothing but troubles;
5) it's just as easy as your average M4, once you accept the fact that AK magazines are shit (regardless of the brand) and won't feed on high RPM builds.. don't ask how I know . Externals are heavier and generally require a hammer + dremel combo to properly fit, but look infinitely better than the average instagram operator M4.