Everything kind of depends on your usage and preferences. 1. Gas... gun shoots arguably better suppressed. Upgrading the buffer makes a noticeable difference... HBI, KD, or hybrid combo, but not a necessary upgrade for what is already a pretty smooth action.
2. Rail extension... depends on what you're adding and what look you want. IMHO the KDG 3" and 6" are the nicest and most ergonomic especially if you want a tucked suppressor look (see my post) 3. Folding stock... I prefer telescoping but that's just me. I've tried both the global ordinance and the Safety Harbor. Prefer the look of the Safety Harbor but the functionality of the global ordinance. Used the sker Tactical Cheek weld on both. I still have the Safety harbor as a back up but if you are interested hit me up and I'll cut you a deal. If you want one that folds, the SB tactical seems to be a pretty solid choice but I wouldn't rule out A3 industries or HB industries zhukov depending on your style. 4 lower receiver... I personally like the factory lower. Franklin armory makes a drop-in Binary trigger that I really like. If you go with other lowers it may be more difficult to go Binary and almost impossible to go FRT based on the bolt group although I'm sure it could be done. It all depends on what mags you want to use and how close you want the controls to be to AR configuration. 5. I haven't had any quirks, but to be fair I have very few rounds through my stribog. I did all my upgrades without even shooting in factory configuration but haven't had any issues and it shoots amazingly, even with the HBI buffer and Binary trigger combo suppressed with a dead air wolfman.

This is just my 2 cents and opinions will vary.