You may only need a cheap one. The timer etc may or may not ever be useful - I keep meaning to buy a timer plug but have never got round to it....30 years and counting 
Check the cleaning details, are there any restrictions on cleaning the pot? There are with my latest one (I have had 2 in 30+ years) but I bought it knowing it wasn't dishwasher safe. Most are, but do check.
Different heat settings Warm, Low and High are necessary... don't get on with just the 2, the Warm is more useful than you might imagine. With it you can switch it on as you go to work, turn it to Warm when you get back and hold it for hours without worrying.
There is Auto as well, Which think it is a good idea but expensive!
Shape of the pot, with/without handles... think about what you want to cook in it. Whole chicken? Get an oval shaped pot, you won't believe how much easier than a round pot it is! Stews only, then a round pot is fine.
Lid... many people say you need a glass lid, so you can see how the food is cooking. I have never bothered, you just leave the food alone for X hours and then eat it! If you lift the lid to look/stir you are supposed to add 15 minutes to the cooking time... makes no odds to me as I lave stuff in for longer than the recipe says, as we are usually out for longer than 6 - 8 hours - which is why I keep thinking about a timer switch.
And then you need a book/website that explains how you slow down normal recipes... usually reduce the liquid content by a third!
That's it, I think!