I have a 124 and a 123 car. I've had five 123 cars over the years, they're indestructible, reliable and parts are inexpensive. In the Dallas area there are jillions of them around, it doen't take much patience to find one with reasonably low mileage in great condition for $2,500 to $5,000 depending on whether you want a 4 or 5 cylinder.
But if you're budget is $10,000, I would chose a 124 car. They are faster, quieter, and better handling. You don't give up any reliability either. In the Dallas area you can get all the '90 and earlier, rust free, low mileage, well cared for examples you want for under $10,000.
Check out the Dallas Morning News. I don't know if they have classifieds on the web, but it would be a good source of a good rust free car.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in