I'm not going to reiterate what has already been said, but I will say this in addendum.
Think about three things.
1. What kind of role do you want to fulfill on the field?
If you want to be an assaulter, it's a far different cry than your average sniper, know what I mean? Your primary gun isn't going to be the same. Think about what you want to do and see what is available.
2. What kind of upgrades are you looking at?
Some guns like the M4 or the MP5 are the Honda Civics of guns. They have laser accessories, grip accessories, RIS accessories, M203s, tac lights, spare rails, iron sights, optical sights, meat tenderizers, AN/PEQ2s, goat locators, accessories that give you a reach around... And others, like the AK-47 aren't so wholly versatile in their range of add-ons. As well, some mechboxes are notorious for sucking at taking upgrades (Read: TM FAMAS)
3. I forget what the third thing was, thank you Sailor Jerry's Rum.
Age Verifier - Lower Vancouver Island
Brotherhood of Nod - Nod Prime || Vancouver Island Airsoft League - President