I would suggest Newegg if you are buying online (provided you live in the US). I have spent loads there, and have only had 1 problem.... My first order had a PSU that was DOA. RMA process was quick and painless, and was the only DOA part I have ever gotten. Shipping is usually pretty quick too....
Anyways... Here is a link for you.... This filters all of their cases to show only Mid & Mini ATX towers, WITHOUT a PSU, and with 1 or more 120 mm fans.
Newegg.com - Computer Parts, PC Components, Laptop Computers, Digital Cameras and more!
For keeping things cool, I would go with 120mm. While I haven't ever given watercooling a shot, I would have to hypothesize that our WC members would LIKELY advise AGAINST WC'ing a rig that is intended to be moved around to LAN party's and such. Again, I personally haven't ever done WC'ing, that is just my opinion. 120mm fans are nice though, since they generally spin more slowly while pushing more air.