--available, reasonable cost, good quality:
Any Mosin Nagant. It shouldn't be long before the Mosin Club gets here, so I'll let them do the explaining on this one.
--available in variation, good quality, likely will hold or increase in value:
The SKS is nice for investment reasons. When my father bought his about 14 years ago they were around $75 for a decent quality one. I just bought one in what I refer to as "reasonable" quality (goes bang, hits what you point at, but finish being total crap) Yugo for about $200.
These are the ones that really stand out to me because they're the ones I have the most experience handling. Take it for what it's worth.