If those are the only issues that car has then you got a really great deal on that vehicle. As previously mentioned, the oil pressure gauge behavior that you describe is normal and the transmission sounds like a pretty typical 190E.

Another item you may want to check regarding that rattle when the engine is at idle is the poly-v belt tensioner shock. The rubber bushing in the shock has a tendancy to break free as it ages and it causes a rattle. It's an inexpensive fix.

I have nothing but good things to say about the 190E. My '92 was my daily driver for 7 years and in that time the only repair needed was a ball joint replacement. Otherwise I just follow the routine maintenance in the factory booklet. I still have the 190E, but my daily driver is now an S420.