Which TF2 Item should i buy?

buy a back pack expander

this ^^

Still useless

You can easily find those hats here: tf2outpost.com

You can also try other trading sites mentioned here: http://wiki.teamfortress.com/wiki/Trading

If it's money you're looking to spend, I suggest the following:
Get a Death Rally 90% off coupon (you can have one from me if you don't have one)
Buy the game for 1 Dollar.
Trade the game for 1 Key (this may take a little time).
Trade the key for hats or for metal (at least 3.33 Ref or 3.22 ref for a quick sell).

Hat prices have gone up, but I just checked tf2op and people are selling their hats for 1 Key (craftable) or even 2 ref (uncraftable), but expect prices to rise even higher in the next days.

If my mentioned trading is too much trouble for you and you have the spare cash, you can simply buy the hats, as you intended in the first place.

... und wenn ich zuerst einen Blick in den Profil geworfen hätte, dann hätte ich auch alles auf Deutsch schreiben können ;)