The 300E (W124) models are one of the most popular. They are fairly easy to work on yourself, and the parts are relatively inexpensive.
Head gasket failure is a problem on some 300E models, and as you pile on the miles, probably inevitable. When it occurs is more a matter of the extent of the care and maintenance during its life.
If your car is overheating and the cooling components are verified to be doing their jobs, then the head gasket is probably on its way out.
The problem I believe is due to the different alloys used in the head and block and over time, their different expansion rates place more stress than the tired head gasket can withstand.
Eventually, you will be due for some light head-related work of some sort, such as valve seals. Most W124 owners when faced with the same, have opted to pull the head and go for a complete valvetrain rebuild while they were at it, so the gasket gets replaced as well.
While on the issue of cooling, if the radiator is original, you will be needing another during its life. The combination plastic/aluminum radiator notoriously cracks at the plastic neck nearly 99% of the time. Mine went out a month ago. Radiator was about $300, and I replaced it myself in a couple of hours.
As far as failures of the MAS or the EHA, (both REAL expensive) I feel that given the W124 threads I've read on this forum that it is not common. There has been a defective wiring harness problem that would have caused symptoms erroneously traced to either of these components by non-seasoned technicians, but considering your current mileage, should have been corrected under warranty some time ago.
What you may experience one day, is problems traced to a little component called the OVP, or Over Voltage Protection relay. This relay prevents voltage surges from destroying sensitive (and expensive) circuitry (like the MAS). When it's not doing its job, you will experience one or all of these problems including rough idling, stalling while driving, problems with hot starts, and diagnostic dash lights (like the ABS) staying lit. The relay is $65 at most, and installs in about 5 minutes. Do a seach on OVP and you will find PAGES of threads on this topic.
I haven't experienced the "gurgle" you refer to in the A/C, but the fan blower does get a little noisy as it ages. Others here have concurred.
The rear power windows may give you problems in the future. The plastic regulator track sliders self destruct and cause the assembly to bind up. Fortunately, the fix is cheap ($2) and can be done at home. Even the whole assembly with motor runs about $140 if you choose not to fool with cracking open the regulator gearbox to respool the cable...not knowing about the $2 fix (both of mine failed within a year of each other) I replaced the regulator assemblies myself.
Other than those mentioned, I think everything else is related to normal wear and tear. Interior bulbs do burn out (even in the Becker radio), and worn suspension components will call themselves to attention with their peculiar noises.
If you think about all that can go wrong, you WILL go nuts! This goes for ANY car you may own, not just the W124. The great thing about this forum is the wealth of information available about your particular model. The unfortunate thing is you get to read about problems others have had with their W124s, so you start thinking that you will inevitably find yourself in the same situation for EVERY case! Not so...
If you peruse through some of these threads you will find that many seemingly catastrophic problems were traced to a worn vacuum hose, a blown fuse, a toasted relay, or even bad gas! But even the simple solutions may take days or weeks
to track down, so don't fret, be persistent, keep checking the forum, and buy your parts from FastLane and PartsShop.
Good luck with your 300E!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car