Ah, I've met my clone!

I also want to keep my car on the road for a little less than forever, and have been fighting rust for about 10 years. Things came to a head when I discovered about 20% of the floorboard was rotten or gone. That's when I discovered POR-15. They make a good line of products to combat rust. (I swear that I'm not a salesman, just a satisfied customer.)

Basically you remove any metal consumed or thinned by rust, and replace with new sheet metal or body filler. Other metal that's rusty but also sturdy can be covered with their paints that seals the rust from the elements, effectively stopping the rust from progressing.

In your case, the rust you describe is very common with the older 240s. The rust may be originating from the inside of the door. In that case, the best approach is to remove the door's inner panel, remove any loose metal or weak spots, clean the heck out of the seam to remove all oil and dirt, and seal it with POR-Patch which is like a caulk. Do this for both inside and outside the door! It dries rock-hard, strengthens the seam and keeps water and oxygen out. You can then leave it as is or topcoat it with matching paint from ipd.

I'm not a pro and am still in the trial-and-error stage (although a little less error these days if I may say so), but this technique has worked well for me.

http://www.por15.com/ Carefully read the instructions and follow them to the letter. Their stuff is not inexpensive, but well worth it.

The good news is that you can save your brick from a premature rust-induced death. The bad news is that since you spotted one area of rust, you likely have others. Jack up the car or drive up some ramps and closely examine it's belly. Also, pull the carpeting and check the floor and rocker panels, the trunk, all wheel-wells (especially the rears) and the engine compartment. Pull off the grille and headlamps and trim. Check the windshield seams. If you're lucky, you won't find a whole lot; if you're like me, your jaw will drop through the rusted hole in the floor. Attack the worst area first, take your time.

E-mail me if you need more info. Good luck. April through October is rust-killing season for me, so your timing is perfect!
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Mike F - 1984 244 DL - 285,000 miles
Original engine, transmission, drive train, starter
Undergoing reconstructive surgery with POR-15