My take as a non-collector.
Collectible, affordable or useable?
In the first category are the rifles/pistols with the built-in gloss of real or imagined history: the lugers, the K98s, the G43s. Anything that's history begins with "There was my Grandad at Normandy..." On the Good-guy side, of course, this would be the war-era US-built Enfields, 1911s, Garands, M1s. Rare pieces like Japanese semi-auto type 4 (or 5?) rifles also. The price for these is generally always high.
Affordable are rifles and pistols that came out of storage at the end of the cold war. These are generally cheap to buy, especially as they first swamp the market or as their ammo dries up. Examples might include: Makarovs, Russian-captured Mausers, Mosin-Nagants, Tokarevs, VZ-52s, Ishapore Enfields, etc. Eventually, their prices will go up. World War 2 japanese rifles tend to be in this group or the one below it.
Useable: What do you like to shoot? People take SKS rifles and 70 year-old MNs pig and deer hunting every year. I used several varieties of C&R Mausers as my back-up hunting rifle for years. They are durable, accurate, hard-hitting and will continue to be used long after I'm dust.
If you're going into collecting to make money, look long and hard to find an item from the collectible group at a steal, so that you can make a decent percentage return in your lifetime, but for god's sake, don't shoot it! If you scuff it, it might become useable, not collectible! Just kidding. kinda. If nothing else, it will be a fun hobby to look.
appraised worth of this advice: less than 2c