First unlock: 1000
Second unlock: 10500
Third unlock: 25200
Fourth: more...
About what weapons to unlock...
The G36C is a really good weapon to unlock, but I do not feel that it gives that awfully much to the SF class.
The PKM as really a weapon to unlock if you are playing as support a lot, it will wipe out anyone with M249 or the RPK-74. Too bad all support weapons are so inaccurate.
The L85A1 is absolutely one of the best unlocks if one is playing a lot as medic. That was my first unlock and I am still playing as a medic mostly.
The M95 sniper is a one shot-one kill weapon if you hit the head but generally sniper takes a lot of training before you can kill good with it.
Oh btw I have 23461points (thats about 94% of the required to get Sergeant).