It sounds like you are going to have a hard time fitting a sump in there. No wider than 13" is not too big of a problem, but if it can only be 18-19" long... that might be a problem. Even a 10 gallon (probably not big enough for your needs) is 20" long.
How about some questions to make sure we are headed in the right direction. Why do you want a sump? What do you plan on putting in the sump? Are you wanting most of your flow to come from the return pump or are you going to use powerheads (my preference) for flow? Do you want the return pump to be external or internal?
For my own personal tank, I am only using the sump for holding the skimmer, heaters, and return pump. Some people try to use the sump as a refuge, but I'm not sure how well the critters survive the trip through the return pump. That's why I'm putting my refuge above the tank, so it can gravity feed to the main tank and hopefully more critters will survive. Also, the new Hydor Koralia powerheads are VERY impressive. DFS is sold out and PA will be getting them soon. A couple of the Koralia 4's should set you up nicely.
Some people also like to grow macroalgae in the sump. It helps to compete with the annoying microalgaes in the tank. Also, if put on a reverse photoperiod from the tank, it can help stabalize pH fluctations.
With an external return pump you will save space in the sump and add less heat to the water. The downside is that you then have to drill your sump as well (not really that big of a downside).
The more stuff you want to do, the bigger it needs to be. So, what do you want to do with it?