From an investment standpoint, your best bet is guns issued by the U.S. military. They tend to be the most expensive milsurps already, however. I think there is potential, also, in British guns. Ammunition will always be available for these, as well.
"Lightly sporterized" guns (that is, guns where the receivers are untouched) are great projects to restore to original military configuration. You have to carefully consider the cost of the gun, plus all the parts needed to complete the restoration, and purchase accordingly. Don't fall into the trap of paying too much.
Milsurps are a great hedge against a future "assault weapons ban." Not only are they unlikely to be touched by such a ban, but their value will go up as current owners of "evil black rifles" redirect their purchasing. I'm telling anyone that will listen that if an AWB includes grandfathering, the value of existing EBR's will go up, but if it does not include grandfathering, the value of milsurps will go up. Personally, right now, I would put half my gun investment budget in EBR's and the other half in milsurps.