If yu are on the last level and you want a bigger or another gun from the shop but you can't aford it then if you finish the game you get all of the guns in the game without paying for them so you get them totally free
Get AK-47 cheap but better in the end when you fully upgrade it.
When my brother anhe bought and I were playing this game he bought an S-System for $10,000 and I bought an AK-47 for 900$. My AK-47 was actually better than his S-System in the end because I spent as much money as needed. So when your trying to beat this game you should try taking this advice. When you get to like the second last mission you should buy a better special weapon and a better secondary weapon also.
P.S. Work as a team. And don't buy a suppresor because they bring down damaga and aggro.