Only the first update weps to each class is unlocked by achievements, bow, jarate and razorback for sniper. Blutsauger, ubersaw and the kritskrieg for medic.
For other weps and also the "old" weps you can get them through drop, for more info on the drops system please follow this link http://wiki.teamfortress.com/wiki/Item_drop_system
Crafting is what you can do with leftover weps you don't want, 2 weapons from the same class and you get 1 scrap, 3 scrap = 1 reclaimed and 3 reclaimed = 1 refined.
You can craft 3 refined into 1 hat, or same more metal to get a class specific hat.
You can also craft weapons, for more information follow this link: http://tf2wiki.net/wiki/Crafting#Metal
And what weapons are best to use? Well that's up to you really, if you like playing as combat engineer you should use The Gunslinger, if for some odd reason like to play as demoknight, you use The Eyelander and The Chargin' Targe xD
All depends on how you like to play =)
Also, shameless selfpromotion: http://www.escapistmagazine.com/groups/view/The-Escapist-Team-Fortress-2-Group-PC