This is quite a complex operation. Setting up a new computer requires specialized (and pricey) tools. It's certainly not worth buying the tools to do this (unless you are a mechanic and going to be using those tools on multiple cars). I'd recommend you bring this to a shop.
Check out the service schedule on this page. For your car there is only one thing to check at 50K. So the cost should be minimal. It appears the next major service is at 72K. You can also use the pre-service section of our site to estimate costs for certain jobs.
Sometimes drain plugs are magnetized, and sometimes they aren't. You can buy magnetized drain plugs if you would like.
As for the fluid level - Did you do the fluid change the Mercedes way? .. i.e. remove the oil pan and replace the transmission fluid filter? (that's right, this trans has a fluid filter, which is perhaps why there is not a magnetized drain plug, or why nothing was on it). If not this may be the problem.