for ease and small size, definitely a mud or musk, no question. A loggerhead musk can be quite attractive as well, with gray skin speckled with black and sometimes some color hues in it.
Anyway, they can be set up very simply. At very MINIMUM you should get a 40 breeder (really you will be much happier if you get a 60 or 75 to start with).
To set up a nearly self maintaining system, you should put in undergravel filter plates and a thick layer of gravel. Run powerheads on this in either normal method or reverse (with reverse you might get less gunk under the plate to clean out every 6 mo.s to year, but that's not too big a deal). Also get a good canister filter like the fluval 404. The cannister will suck up the junk and you can clean it every few weeks very easily, while the gravel will run a great biofilter to maintain your water quality.
Put in plenty of aquatic plants.
Put in some rocks/driftwood furniture. Make sure he has a basking spot he can climb on to get totally out of the water. Make sure he has a spot to hide in the water (a half of a terra cotta pot, or the whole one buried halfway in the gravel on it's side works well).
Get it all set up, put on a light set (either a flourescent fixture with a reptisun 5.0 AND a dome with a household bulb for the basking spot to get into mid-high 80's, OR just a dome with ceramic socket to hold a mercury vapor bulb which is both heat and uvb in one), put your lights on an outlet timer for 14 hour days. Turn on filters. (you don't need a heater with these species unless you keep your house below about 72 degrees). Let the tank run for at least a week with a few small fish in it before you add the turtle.
Now, feed your turtle ONLY in a separate container from day one. It may take up to a few weeks for it to start eating this way (use a rubbermaid dish or something with a few inches of water so it can submerge and eat) but it will not starve to death. NEVER put food in the tank. This way you can keep out 90% of the waste of turtle feeding and even it's bowel movements will often come in the feeding bin after it eats, and not be in your tank.
Your tank (provided you have about 40 gallons or more of water) will just need to be topped off every week or so with fresh water (due to evaporation), your cannister can be cleaned every 4-6 weeks, and your gravel vacuumed (they make great attachments to the sink called a python that will let you do this easily) every 3 months or so. Every 6 months or year you can do a more comprehensive cleaning.
Kind of labor intensive and maybe a bit expensive to get it all set up, but when you are done it will be SO SIMPLE to maintain, and it will look SPECTACULAR. After well over two decades with turtles I garauntee you will like the animals more if their cages are beautiful and easy to maintain.
