How reliable would an early 90's mercedes sl be? I have never had much experience with mercedes.

I would pretty much stay clear of the early 90's SL's and look more towards the late 90's and here's why. These(the 129 chassis) were introduced in 1990 and had several issues with them such as the soft top control units and the hydraulic cylinders that operated them , engine wiring harnesses along with pricey services since they use twin ignition caps and rotors accompanied by expensive spark plug wires should they go bad and they will . By the late 90's the bugs have been worked out and they became a very nice vehicle

The early 1990's SL used an environmentally friendly wiring harness which completely disintegrates as the vehicle approaches 100K miles. This is very expensive to replace. Also, the body style that began in 2002 has a retractable hard top as opposed to the previous models' two-top system. While that doesn't necessarily speak to reliability, convenience should be considered as well. If you had a pre-2002 SL and did not put the hard top on in bad weather, reliability would eventually suffer because the soft top would incur wear and tear and leakage.