Welcome, you have asked some really good questions.
Luger "shooters" of WWII vintage are IMO a great first pistol from a standpoint of price, availability and quality. A WWII vintage shooter can be had for around $1,000, give or take a few hundred depending on finish, if/how many parts match, overall condition. If you have the patience it is best to buy a book or two first. The nicer books like Still or Gortz & Sturgess cost hundreds of dollars but that is very cheap insurance vs. making an expensive mistake. The biggest factors in getting a decent shooter at a fair price are IMO knowledge and patience.
Lower end collectibles (a collectible Luger typically has original finish and all or almost all matching number parts) start around $1,200 or so, my cheapest collectible was $1,250.
This forum and the other forum
http://luger.gunboards.com/forum.php are excellent resources for buying. Auctions can be too, but require caution. The local gun shop/gun show is IMO not a good resource, few choices and they mark up too much. Simpsons web site is another dealer option, they do get shooters from time to time but they go very fast - within hours of posting - so you must check their 'new items' listing frequently.