HL:Source will have upgraded graphics and physics. It will not receive the upgrade both CS and DoD have gotten.
About buying stuff in CS, it's really simple. It uses a branch system. 1 branch for weapons, and 1 branch for other stuff (like armor, grenades, bomb defusing kit etc.)
The weapons branch is devided into 5 parts:
1. Pistols - Desert Eagle, Glock etc.
2. Shotguns - Pump action, automatic
3. SMGs (Small machine guns) - MP5, FN-P90 (RCP-90 form GolderEye)
4. Rifles - M4 Colt, AK-47, but also (automatic) sniper rifles
5. Machine guns - Think Rambo
In each of these branches you can choose, or buy, one of these guns. You can only carry one pistol and one primary gun (branch 2/3/4/5) at any given time.
The better the gun, the more it costs. Money is allotted to you when you kill someone or complete the mission. This can be rescueing the hostages or planting/defusing the bomb. This depends on which map you're playing. So if you wanted to buy an MP5 SMG, you would need to press branch 1, branch 3, object 2. (with the standard keys that would be: "b-3-2") You can also use your mouse and the GUI but as you play longer and become accustomed you want to speed this progress up by remembering the combinations.
Things that you need to buy every round as well are "armor and helmet" (branch 2, object 2). It is expansive ($1000) and it doesn't make you a terminator, but it gives you the opportinuty to receive 1 extra bullet before you die. And in a game where 2/3 bullets are usually lethal it can be as big as winning or losing the round. Grenades are also VERY useful. You need some practise to finally get the hang of it (especially flashbangs) but you will have a huge advantage once you do.
Other things you need to be aware of is that headshots will kill instantly most of the time, especially with the higher caliber guns like the rifles and Desert Eagle. When you're moving your accuracy drops drastically (that's why you see everyone ducking as this increases it) and some players have above naturals skills. Thing is, they've been playing this game for 4+ years and know every nook and cranny of it. Stuff like hearing which gun you're using, how many bullets you've shot so they know when to come after you when you're reloading and knowing every pixel of the map (huge part of the game).
