From having toiled as an auto mechanic fot ten years, this I have learned :
Assure that the most expensive item is still serviceable, and will last long enough to justify repairs.

Your car and mine as well are "borderliners", IMO. Cars between 12 to 15 years of age - Mercedes 15 to 20 years.

So, after you find that the body is good and the engine has good compression and is not making horrible noises, or losing oil and coolant, then start the diagnosis.

Now, at this point, having already decided to repair the car, borderline or not, assure that maintenance items are still serviceable(filters, plugs, wires, oxygen sensors(and others) belts, hoses, the valve adjustment(if necessary), the lubricants and fluids)..And don't forget the battery and charging system..

Most likely, not one penny has been spent, unless you pay someone to do this, which is about one hour or so of labor.

Please do not guess ! Having pulled the plugs, examine them.
wet - gas likely OK, ignition may not be OK
dry - gas problem, ignition probably OK
Also when the engine is turning over, observe the valve action
if dry plugs, trace back to the source, Hell, even the tank could be empty, I have seen this happen. What is more likely is a bad fuel pump relay. I almost that these fail so much after the vehicle is 12 to 15 years old, that they should be a scheduled replacement item.

The idea is this : Other than the one hour diagnosis, do not replace anything until you know for certain it is bad. And always start with the cheapest item.
BTW, forget about the previous expenditures - most of these are maintenance items for a quality vehicle such as a Volvo ( my daughter owns one).

Adrian
Two Diesel caddys and one Mercedes 300D