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Purchasing a Mercedes Benz with 100k miles is not a problem.
Unlike several vehicles, Mercedes are built for life. I constantly see Mercedes in the shop with 200-300K miles 15-25 years old still running strong.
You will want to have the vehicle inspected by your local Mercedes dealership prior to purchase. Do not trust any regular repair facility. Unless you take the vehicle to a Mercedes specialist, you should only trust the dealership.
Also ask the seller for copies of the maintenance records before you have the vehicle inspected. Give these records to the technician and service consultant to make sure that all of the maintenance is up to date.
If everything checks out then buy it.
I have not seen many issues with the 325ci convertible.
Visually you should make sure that the convertible top is in good shape. Check to make sure there are no leaks or seperating seams. If you dont feel comfortable inspecting it, then you can allow the dealership or a body shop inspect it for you.
Beyond that my advise is the same as the Mercedes.
Generally both BMW and Mercedes hold up pretty well over the years. I tend to see Mercedes last longer, however I do see a few high mileage BMWs in the shop each month.