So... time has marched on and I have word back from the machine shop that the cylinder head for my 300E is in reasonable shape - just needs new valve seals and maybe valve guides. He is still checking the bottom end but doesn't believe anything will be found.

Since I have the transmission out of the car, I am contemplating some service on it. It has two problems. First, since I bough the car at 135k it has always stuck in first gear for the first few blocks of driving in the morning before making it out to second. It has become noticably longer now at 144k. Once it unsticks from first, it drives well for the remainder of the day.
Second problem is that if I accelerate hard to pull out from a stop, it slips badly (lots of noise but little acceleration). My normal driving habits make this a rare occurence but I always have it in the back of my mid that the car is not to be put to the test when pulling-out on a busy road.

The transmission shop I've always used (one of the largest on the east coast) said they wouldn't work the transmission until I had the car re-assembled so they could diagnose if the problem was internal or not. Seems reasonable but at the same time I would be eager to replace items either related to the issues above or just those that are normally wearing out at this stage. For example, I've seen comments about B2 piston replacement.
What is considered preventative maintenance at 144K? I suppose I could at least do the filter and pan gasket. Is that a DYI job?
Any thoughts? Is it better to leave well enought alone? I mentioned resealing the transmission and they felt that if it wasn't leaking already they wouldn't recommend a reseal (what isn't broke, doesn't need fixing...)