Well since you asked and, since no one really answered what you asked, allow me. You may want to buy some gun valuation type books to begin with. The 2005 Standard Catalog of Firearms (The Collectors Price and Reference Guide) is a fairly good one to start with. Read the articles in it. It will point you in the right direction. Just going out and buying well made firearms from good name manufacturers is not the way to go if you are primarily concerned with guns as an investment. Many guns that come out, that are earmarked as collectors editions, actually go down in value over time. Some restored firearms are almost worthless to collectors as investments yet, others are quite valuable: for instance a refinished Colt Single Action Army revolver recently sold for $11,200. It had a provenance (a history of its origins) attached to it. It had belonged to Wyatt Earp. Another revolver, this one was nickel plated S&W model 3 with only about 60 to 70% of the original nickel (pretty bad shape for a nickel plated gun) sold for $385,000 at auction in 2003. It too had a special provenance, it was given to Bob Ford by Jesse James, Ford later used it to kill Jesse James. While some model 3s can sell upwards of $14,000 most are in the range of a thousand to a few thousand dollars. A firearm with good provenance makes a good investment as long as there is an interested market.
Of course, many firearms without a special origin are also fairly good investments. Old rare guns that are in demand can bring nice money. New rare guns in demand can do likewise. Demand can change at any time, but collectible guns have, in general, kept going up in value.
The 2005 Standard Catalog of Firearms can give you a good look at what guns are worth, will give you the basics on how to grade them, and will give you some insight into the world of gun collecting.
Good luck with your investments.