The biggest problem with most new riders and their bikes is fit. I picked up a Haro MTB as my second bike about a year ago. I liked the pike but the wrists would kill me after only about ten miles of riding. 5 miles on the dirt was all I could do even with bar ends added. I took it to the LBS where I got my road bike and with a different stem and saddle adjustment I was in business. I now have a Trek 800 and the first thing I did was duplicate the set up of the Haro and 20 to 30 miles is very easy on most portions of the body, if I don’t fall down. I don’t even need bar ends on the Trek. SO before you do anything or buy anything, go to a LBS, and see what fits you. Take measurements and then look for something. You haven’t mentioned price yet and sometimes a new bike can be had a lot easier than a used one. Remember to ask yourself if the person selling you their used bike has a reason. And then ask yourself if you know enough about bikes to know that you can discover if that reason in mechanical?