Do a lot of research, and make sure the one you get is one that has everything you need. No need to buy a tablet which costs more for features you don't think you'll need.
Also, don't think that because you have a tablet, it will make your art better. If anything, it will take a lot of getting used to, and will most likely be difficult for the first couple of weeks you're using it. It's just something you need to ease your way into, so don't be frustrated if your first couple pieces come out looking like shit.
Well there are 2 things to look for when looking for a drawing pad.
1) Active area size (size of the active area you can use)
2) Pressure Levels (sensitivity)
I use a Medium Intous Pro and im quite happy with it, also used a Intous 3 before that. I havent used anything else and i doubt there are alot better ones which are reasonable. If u feel that those cost too much then you can look for another series, but i woudnt recommend any other brand.
Also buying a used one isint a bad idea and the older models like the Intous 3 are cheap if you can find any.
If you get a good pad it will last for years, maby if you use it much the only thing u need to replace are the pen nibs. Also if your intrested in wireless option then its not good from my own experience.
One thing i had problems was that its hard to get drivers work with windows 8, they work but the sensitivity is way off. There are guides and such which help you fix it.
Are you any good at drawing with a pencil on paper? If so, get one. If not, don't.
I can only suggest for you to go to the biggest stores you can go and check their product ranges....
Talk with the staff. They might help you.
I think 2 couple days ago, I saw some tablets used to draw.. It's pretty good and the price quite expensive for me.. If you want to buy a tablet, make sure you ask the staff the advantages using these tablets.. I'm pretty sure that they knew more than you about the tablets.. Good luck.