Hi Lars, welcome to the world of harp! I will try to answer some of your questions, though I know there are many others here who will also have input–and better than mine! 🙂
Different harps have different numbers of strings for several reasons: size, sound, cost, style of music played, etc. Some people want a small, portable harp to take anywhere they go. Others want a large floor harp for folk music. Still others want a large pedal harp for complex classical music. All of these have different strengths, uses, and costs.
Alison Vardy has some good info on her site about the history of medieval harps.
http://www.alisonvardy.com/harp-info/medieval-harp-history.htm
There are, of course, many other resources out there, and a search of these forums would probably help you as well. I hope you enjoy find what you’re looking for, and enjoy the harping adventure!