mmhh Christmas lunch ;-). I'm actually having a hot leftovers lunch today - I'm freezing here as usual, even wearing fingerless gloves...
Anyway back to slow cookers.
Two heat settings: yes because you want on 'high' when you're preparing things. you also want the option to halve the cooking time by putting it on high.
Mine has an 'auto' setting which means I switch it on it goes on high and then automatically switched to low after 30 mins or so (when I'm long gone).
Oh and mine has a pyrex glass lid - perfect for checking progress without lifting the lid (definitely a no-no in the SC world, the heat escape would prolong the cooking time by at least 30 mins....
And get one with plastic casing, not stainless steel even if it look nicer - it's safer as the metal could get very hot.
[edited later: ohh I see that that's not an easy option these days, they all seem to be SS anyway - please ignore!!]
Long cooking times and timers: No essential as Vicky said, maybe attach a separate one. I didn't know you could get SC with timers... The point about slow cookers is that they use about a light-bulb's worth of electricity (I think / I hope!) and CAN therefore be left on that long. Your food's just going to get a bit more stewed
and might have a bit of a crust on top.
Here is some buying guidance:
Slow cooker basics - Before you start, read more about using a slow cooker
Slow Cookers: Information and how to choose and buy slow cookers
Anyway I'd definitely vote for you adopting your mum's!! At least to start with.