My belief is: Buy what you need, you do not need: helmet, 2 weapons, a pistol, a new melee weapon, body armor, new clothing, etc. Waste.
I stick with buying the Uzi cheapest automatic gun (700GP), it have good stopping power, very good ROF, and fairly accurate at CQC and MC. (Close Quarter Combat and Medium Combat)
I also stick with the G36E, it is a GREAT gun, great stopping power, accurate, and reliable, it a great gun, more expensive but you'll be just fine.
Stick with 1 Day weapon, the next day, you can 'rebuy' the weapon, adding 24+the extra hours you put on the weapon.
Mod only what you need. NEVER buy silencer, a total waste, don't buy the scopes, unless you need it, sense most weapons can hit targets without it.
Don't mix two classes that does not go well. Mixing an MG and a sniper does not mix will, sense the MG have a 2 second 'loaded and ready' animation and the sniper is Long/Medium range.
Using a Sniper and an SMG is fun. Do not buy another pistol, I can still kick people ass with just the default pistol. Don't buy a new melee weapon unless you're planning to spend a few hours, around 5+ or more on knives only.
Don't buy grenades, don't buy mines, smoke, flashbangs, if you can fund it. Fine, smoke tend to be wasteful I can still see your name behind that shit, flash grenades are okay, if you can fling them out. The only grenade you should get, it's the fire/gas one and the fragment grenade (bigger and wider boom).
Don't buy clothing, such as arctic vest. All that stuff is plain useless.
Don't buy helmets or 'glasses' or even masks, the masks barely help much, the helmets does not protect much.