Here a list of WWII rifles you can collect:
American
-US. Rifle, cal. 30, M1, M1C (Garand)
-US. Carbine, cal. 30, M1
-The Johnson rifle
-Springfield 1903, from A1 to A3 version
German:
-Mauser 98K 7,92 x 57 (the cheapest version are the one built by CZ)
Italian:
-Manlincher-Carcano mod. 91/38 cal. 6,5. I don't know availability of ammos. But if you collect only, no problem. There is also a short version used by cavalry. Anemic ammo but if in good conditions they are accurate.
Russian:
-Moisin-Nagant. Cheap, but quality can vary a lot. Cheap also the sniper version.
Japanese:
-Arisaka. Interesting for collection. Poor quality and accuracy. Excellent bayonets.
England & Commonwealth:
-Enfield Mk1, Mk4 cal. 303 British. Also interesting the T (sniper) version of this rifle. Plenty of versions. Interesting also the Mk.5, so called Jungle Rifle.
-Enfield P14. Version in 303 Brit. of the P17 of the Sgt. York.
There are also non belligerant WWII rifles.
I feel that the most interesting could be
-Swedish M96 Mausers in 6,5 x 55 Swedish. Superb steel, actions and barrels, even in cheap models, can still offer good accuracy. Ammo available, and SHARP.
-Swiss straight-pull Rubin-Schmidt Rifles. I think that the K38 model was the one in use during WWII period. Cal. 7,5x57. Accurate, powerful, but ammos aren't so easy to find.
I hope I have helped you.