To begin with, I can relate to your situation. I work for a software company in a niche market that is extremely challenged. I also have college age kids and am in a very similar struggle and unknown area. So, best of luck on that front.
To your car item by item:
1. My 240D went over a half million miles before ANY front end component was replaced. These cars have an incredibly durable front end. For this reason, I would suggest that you replace only the components that are loose. A competent front end man can push, pull, pry and prod to determine what should be replaced. These components are expensive, and it's just a waste of many to replace ones that are still tight. If a component is still tight, odds are it will stay tight for a long time to come.
2. The calipers are very rebuildable. rarely do the hoses need changing, and the bearings should need no more than cleaning and repacking. The rotors on these cars stay true and rarely need turning or replacing unless the pads are allowed to wear to a point of metal to metal contact. If you replace the sensors every time the light on the dash will come on and warn you long before metal to metal contact.
3. I have NEVER found the need to replace ANY rear suspension components except subframe bushings and sway bar links. The sway bar links will start to rattle and they are inexpensive and very quick and easy to replace. The only thing about the rear suspension you may get into is adjusting the valve link, replacing the accumulators or replacing the rear shocks.
4.With relatively new accumulators, that should be no problem. Flushing the system would be very wise preventive maintenance. These systems are durable for the most part. Suspension bushings, etc. would be a rare failure on these rear suspensions. Again, push, pull, pry and prod to check everything. Make sure you are using a tech that is not just trying to run up a big repair bill. I have seen some situations like this.
5. The acceleration problem is most likely due to the aneroid adjustment. I have never done this, but there has been much instruction in the Diesel Discussion Forum regarding this. You just need a savvy MB guy to check this out. It should not be a big deal. There's not much to go wrong with the linkage itself. In fact I would be suspect of a tech who told you that this was the problem.
6. Lack of fuel flow could account for this, but if this is the problem, most likely replacing the two filters at the engine would correct it. Since you don't drive it alot, maybe algae is a factor. If so, it could probably be corrected with bioguard alone.
7. Never hurts, this is just good, wise preventive maintenance.
8. Again, it never hurts.
9. Refer to 8.
10. Refer to 8.
11. If the door locks work well, even after the car has set for a few hours, the likelihood of a vacuum leak anywhere is almost nil.
12. Always helps for them to look good. It keeps you willing to take care of items such as in items 1. through 11. above.
13. Refer to 12.
The conversion to 134 without flushing may not be a big problem as long as you put in the correct amount of ester oil. These cars have marginal a/c systems, however, particularly in wagon form. The wagons have all that extra volume with no extra a/c capacity. I understand your tendency to use 134, but I don't see how you can possibly have enough a/c capacity with 134 to get the job done in Dallas during the months of July and August. R12 is coming down in cost since the cars that use it are hitting the wrecking yards every day.
The big item with the a/c as you have already pointed out is the evaporator. If you were to replace the evaporator, thoroughly flush, put in a filter drier, etc., putting R12 in the system would probably be about 10% or less of the overall cost and would give you the best possible chance for staying comfortable.
I understand your enthusiasm for your 123 wagon. These cars are fabulously built automobiles. They cost lots of money new because it took lots of money to build such a solid machine built for long term durability. To buy an equivalent new car would be something like $50,000 or more. You can do a lot of things to this solid old ride for what only one car payment on a new wagon would cost.
Best of luck with the car and the airline industry.
BTW, I'm trying to get another 6,000 miles in before March 15 to get American Advantage Platinum for 2002. With some luck, American will get past their challenges so my American Advantage miles will be worth something and more importantly, you can still have a job.
Keep that airliner out of the trees,