as to "what to get" to complete the collection...
that all depends on how "complete" you want to be, whether you want to limit the collection to only certain national origins, and how much money you have at your disposal.....
so here's MY list with dividers for "should get", stuff that was second/rear echelon etc, and "how much cabbage you GOT jack?" (note: since this is the rifle area i'll limit to rifles and carbines)
US:
Rifles NEEDED
M-1 Garand, M-1 Carbine, M1903(A3/A4)
rifles of secondary usage
1917 enfield
things limited by cash flow/rarity
M1941 Johnson Rifle (Mil-tech has some of these but the "refurb" they do kills alot of collector value)
Japan:
Type 99 and 38 Arisaka rifles
UK:
SMLE (No1 Mk3), No.4 Mk1 and No.5 Mk1 Enfields, with a P14 (upon which the M1917 "enfeild" was based, difference being chambering) thrown in to Represent the homeguard (if you can find one that is)
Germany:
Must have
K98 Mauser
if you can afford and/or find
Gewehr43 and K43 semi-autos (aka "hitler's garand"), STG44, FG42
also remember that the Germans never tossed away anything that actually worked, even if it WAS the enemy's gear, so it was entirely possible for member of the wehrmacht or a volksgenadier to be armed with a Garand or SVT-40
Russia:
whichever example of the Mosin-Nagant family you're most comfortable with/can find easy but a 91/30 is probably most representative.
Then there's the SVT-40 (or slightly earlier SVT-38)