Welcome from "Down South"..!!
Also, which trim level comes with parking sensors? Or is this an add-on only?
Assuming the order of expense is as follows: standard, sport, edition 10 and grand edition, I find it strange that the trim levels seem messed up. Eg standard trim comes with climate control (which is better than air con) but other, higher trim options, only come with air con. Or some come with heated seats and air con but not climate control.
If given the option between a low mileage older car (say 55k mile 2004) and a newer higher mileage car (125k mile 2006) is there any strong reason to go for the older car? Naturally, I'd prefer a younger car and the higher mileage shouldn't be an issue but would I be making a mistake?
Similarly, if given a choice between an older, higher mileage 280 and a younger, lesser mileage 200 would I be making a mistake in going for the 280?
Just looking through Your good questions:
Having met with some SLK owners yesterday, rust is a bigger problem on the R170's & a lot (most?) have been re-sprayed, but the rust will be back...
It's just a case of do You like the looks of a R170 or a R171 (The R171's also got a face lift in 2008)
If Your don't want the additional speed of a V6 engine (280/350), getting a SLK 200 would make a lot of sense & will save on running costs.
I also think when looking for used SLK's, the 200's come in more colours? (V6 SLK's tend to be Black/Silver..!!)
As already advised, the Manual gearbox is very rare in the UK - Try out a Auto SLK 200 & see what You think?
The Flappy Paddles where some think like a ~£120 option, but don't have to be used
Here in the UK, Ford/Peugeot/Nissan ect have trim levels:
Mercedes on the other hand had a standard SLK that came with a fancy folding roof, cloth seats & a steering wheel - Every thing else was a option that the first owner had to pay for....!!
(I jest a little, but You get the idea)
While looking for mine, I came across a 2008 SLK 280 that had £12k of options...
In My mind Heated Seats are the No.1 option You need to look for when looking at a used SLK.
Then the nice to have options are Air Scarf, COMAND (Better radio/Sat-Nav) Parking Sensors, ect..
Plus You also need the bit of Perplex between the two role over hoop's which is very important for driving top down in the UK...!!!
As for mileage - It's all about how well it has been serviced & hard to say if a 50k car is better that a 100k car, with out seeing both...
Also checking out the MOT history can give a clue to how well the car has been looked after:
(Then click on the "History" drop down box)
Good luck in Your search & feel free to come back with links to cars on Auto Trader ect, and I'm sure We will chip in with our thoughts