Only the usual things as with most cars – check for a decent service history from a Merc dealer or Smart specialist. And make sure you get a top of the range model with a full auto box. Having a LHD one doesn't make much odds when it comes to driving it as it's so small and insuring it wasn't a problem either.
I'd make sure you've got a Smart specialist garage nearby rather than having to pay Mercedes mechanic prices 🙂 They're just too weird for a 'normal' garage to work on (there's no sump plug for example, to drain it you have to pump the oil out of the dipstick hole!).
We had one for just under a year in which time it managed to run up over £1000 of repairs: steering column, exhaust, air bag, alternator etc. Parts all cost as if they are gold-plated and as before all Merc mechanics cost big bucks.
Have a drive in one first 'cos the gearboxes take a bit of getting used to and the change can be a bit slow, although you can get them 'chipped' to get more power (50%!) and faster changes. They've also got the most comfortable seats of any car I've been in 🙂
Have a bit of a dig around for some Smart forums as there are lots out there able to offer advice…