if your going to try to bring down a deer your going to want to spend a lot more then $40, with that said there are plenty of capable crossbows out there and a few different set ups. Your going to want to look at the two main types of compound and recurve both are capable of taking deer, really it's a personal reference, which ever you choose you should get a draw assist it makes string tension the same on the entire bow string and can make it more accurate. Really you don't need anything too fancy if your doing target work or deer hunting, really it's all up to you...
"come on you sons of b***hs! do you want to live forever?" Sergeant Daniel Daly, USMC, Belleau Wood France June 6th 1918
"You can be as mad as a mad dog at the way things went. You could swear, curse the fates, but when it comes to the end, you have to let go" Benjamin Button