You've already received some pretty accurate responses from a number of posters in this forum, and I too, share the same sentiment...
...many of us were smitten by the opportunity to own an older, legendary MB vehicle at a good price (thanks to depreciation), only to find out that old cars wear out.
Many MB parts are expensive, partly because they are designed to last for more than a decade of use. Labor costs are daunting, and the same skills needed to maintain a 2002 MB applies to older models too.
This forum has a large share of its members who own older MBs, enjoy them, and are looking to defray the cost of maintenance by learning how to work on them. It's easier if you have a standby car that you can drive while you're baby is on jackstands.
This forum is valuable in achieving that, and I can attest to saving $$$$$$$ by doing the work myself. Sure, I would like to be able to just drop it off at the local MB shop and say "fix everything that's broke". Since my last name is not Gates, that's not an option.
So if it takes me a week off and on to replace an alternator, so what? It cost me $130, and not $750!
Hang in there, as there are many 300E owners here, and the problems you've described sound like common issues, not catastrophic ones. And as said earlier, what the PO did (or didn't do) could take several years to cease erupting into problems...my 300E has only recently stopped breaking stuff, although now the wear items are longing for attention. So it's a work in progress. But it runs and takes me to work daily, andlooks good doing so.