As for fitting the motherboard in the case, be prepared to use a drill, as the holes would be in a different place on pre built pc's.
For cases, you are going to have to make the final desicion as you are the one that is going to have to see it every day. So get one that you really like the look of, even if it costs a bit more. Look at review's for it as well. I would recomend a all metal one, preferable aluminium (due to it being lighter,) as the front of my Antec case is plastic, and I broke it, so I had to take off the things that lock it in place to get it to go on.