First off, welcome!
Secondly, if you want individual pieces, check this site out. I’ve bought a lot of things from these guys and always been happy with the service, shipping, and products –> http://miniaturemarket.com/axis___allies
Lastly, here’s some general pieces I recommend you try and pick up (I won’t list the lower-end, filler stuff):
Allies:
-Guards T-34/85 (SU, Set II)
-Fanatical Sniper (SU, Set II)
-Matilda II (UK, North Africa)
-Spitfire Ace (UK, North Africa)
-Valentine I (UK, 1939-1945)
-M1 81mm Mortar (US, D-Day)
-“Screaming Eagle” Paratroopers (US, Set II)
-“Screaming Eagle” Captain (US, Set II)
-Resourceful Hero (US, D-Day)
-M24 Chaffee (US, Set II)
Axis:
-King Tiger (GE, Set II & 1939-1945)
-Grizzled Veteran (GE, D-Day)
-88mm Flak 36 (GE, D-Day & North Africa)
-Sturmpanzer IV “Brummbär” (GE, Set II & 1939-1945)
-Messerschmitt Ace (GE, North Africa)
-Panther Ausf. D (GE, Reserves)
-Sd Kfz 234/2 “Puma” (GE, Contested Skies)
-SS-Haupsturmführer (GE, Base Set & 1939-1945)
-Panzer IV Ausf. H Commander (GE, North Africa)
-Veteran Fallschirmjager (GE, North Africa)
If you have any questions or anything just ask! Also note that a lot of the cards have been revised since their original printing (found here: http://forums.gleemax.com/showthread.php?t=1089055). Of all the units I mentioned above, if I could only choose 5 to recommend, I’d advise the Puma (it just got recosted down to 17 points and rules), the King Tiger (because it’s now 62 points and pretty unstoppable), the M24 Chaffee (as it’s my favorite Allied tank for the amount of ass I’ve seen it kick), and the combination of the “Screaming Eagle” Captain and the M1 81mm Mortar (because the mortar can fire blindly at anything within 4 hexes of line-of-sight that the Captain has… and as he’s a paratrooper, that’s pretty deadly).