Hello willybe and welcome to Tnet! You are asking the question that so many others have asked and I would have done the same IF I had known of this site. It all depends on what you are going to be searching for, what part of the country you live in, what kind of soil you will be searching, will there be power lines overhead and other questions like this.
Basically, purchase a well respected brand and that would mean they have an awesome customer service plan and have had it in place for many years! Also look under the brands section and then for the specific brand/model you are looking for and then ask your questions there and you should have plenty of responses and info come you way.
I've used Whites Detectors exclusively since starting back in the early 70's save for once when I purchased the Fischer Gold Bug and never found anything with it. I started detecting for coins in town with a White's 6000D Coinmaster that once it was setup properly I KNEW what was under the coil before I dug the target. Do your research well, purchase as much machine as you can afford and understand that ALL machines have a learning curve usually the more expensive the steeper the curve. Be sure you really want to spend many hours out detecting and finding nothing but junk that you take home with you and put in the trash. Note here: In no time at all I was finding coins with the 6000D and that was before I learned how to really set it up so it would find everything and knowing what was under the coil before I dug it. Also, dig holes carefully so no grass dies and cover the holes after making them as nothing will loose us and you the right to detect faster than leaving holes and dead grass.
Welcome and good luck.............63bkpkr