I have a 1989 300CE. They are great cars when properly maintained. I had a similar starting problem. My fuel pump relay was replaced and the injectors cleaned and that solved the problem. I have never seen a badly rusted 300CE, I think they are fairly resistant to corrosion but there may be exceptions. These cars start in 2nd gear so they are a bit slow off the line unless you shift manually, accelerate hard, or have the transmission modified to start in first. I prefer a stock unmodified car so I would avoid the example you are looking at but if the price is right get a pre purchase inspection before buying.