I have bought a number of student properties over the past 12 years, only one of which was tenanted. I had no problems with it - in fact it was easier since the tenants, the furnishings, kitchen appliances etc. were all in place.
You just need to get a buy to let mortgage as usual - I would use a broker to find a good deal.
You will have found out if the tenants are taking care of the place when you looked around it. Check the tenancy agreements with your solicitor, find out if the rent is being paid, and remember to sort out the deposits which will be held by the present owner.
Once you have bought the house you will need to arrange to visit occasionally to check everything is going ok.
Chris Bell
http://www.studentpropertytycoon.com