You need to make that decision for yourself. You are missing a lot of nations rifles. Japan, France, China and Italy being the major powers.
Finland, Poland, Switzerland, Netherlands, Belgium,
Canada,
Australia,
Sweden,
Norway,
Denmark. Bulgaria being among the minor powers. Granted some of them remained neutral throughout the war but the
Swiss K31 is an absolute must in my opinion.
Japanese and
Italian rifles are still bargains despite some serious movement on the Japanese end. Poland is already expensive, Norway, Denmark, Belgium and the Netherlands almost impossible to get, China is cheap but try finding a decent one. Canada and Australia often get lumped in with the
UK and they did you a lot of the same equipment. France and Finland are reasonable but moving up. Switzerland's days of cheap K31's are unfortunately over. They jumped to $600-$700 it seems but you still might find a bargain or two to be had in the $400 range if you are persistent and lucky.
If you want a true basic WWII collection you have to include the
French, Japanese and Italian weapons as a must. I'd also push China pretty hard too because their war actually started in 1937, they fought just as hard as everyone else and are just largely forgotten by us here in the west.