My wife has had an R171 SLK350 for almost 5 years and it's been a great car and great fun too.
As far as buying tips go, I'd say look for the highest spec car you can afford, and in particular make sure you get one with AirScarf (if it has that it will also have heated seats) as it makes a real difference as to how many days of the year you can comfortably use it with the roof down.
As far as checking the car over, obviously make sure everything works properly. Check for signs of water ingress around the roof seals, including looking for evidence of the spare wheel well being or having been wet. If it looks like there is or has been a problem then walk away as the roof motor doesn't like being damp and is costly to replace. Check also that the heater fan works properly. Blocked drains can cause this to suffer water damage and fail.
One other thing is that when checking the tyres over remember that the fronts tend to wear the inner edge ahead of the centre and outer, so at casual glance can look like they have a reasonable amount of life left when in fact they need (or are close to needing) replacing.
