OP's comments are bang on, just wanted to add what I've learned from the 4R100 trani (updated version of what's in yours) in my '01 350:
1) ATF changes are spec'd at 50,000km/30,000 miles. You really need to change the pan filter on the same sched too, as it can plug (not a screen, like in the old daze). If DIY, make sure you change out the filter seal, if you re-use the old one the trani can end up sucking air (didn't go there myself, just happen to know a really wise wrench).
2) Had an OE labelled Magnefine filter in my return line. Cut it a part when I replaced it - liked what I saw, and the same wizard I know gave the bypass design in this filter a thumbs up - it's rated to bypass at around 5psi, if the filter body plugs. Worthwhile filter.
3) When I had the pan off mine, had my buddy braze on a drain plug - I can now get about 5 quarts dropped at a time - really convenient.
4) My van had mostly decent fleet maintenance, but the trani had not been serviced "in a while" - it is one thing that people tend to ignore. If there is some sludge in yours, I'm hearing some not great things about System flushes on old trani's - it can stir up TOO much junk and cause a problem. I wanted a drain plug on mine anyway, but what I plan to do now is about every 1000 miles, drop 5 quarts out and keep doing this till mine runs clean.
Poor man's flush - yes - but I'm hoping between that and the Magnefine, nothing gets plugged up.
5) The air/oil trans cooler mounted at the front is often susceptible to plugging up with sludge, esp. if the ATF has not been changed on sched. A trans shop should be able to assess flow rate on this if you are getting serviced. These can be flushed on their own, or replacements are not that much $.
6) I've recently got a Scan Gauge II, and was able to program it for Trans Fluid Temp, and Engine Oil Temp. If you are planning any heavy hauling (even just regular loads/stopNgo traffic can be nasty), a Trans Temp gauge is always a good idea. Not sure if this Scan Gauge would work on your year, but there are many dedicated Trans sensors and gauges that would work.
Any used vehicle is going to take some work, check it out the best you can, from there it is a bit of a c*apshoot though. Best luck, I know I'm glad I bought my E350. Rgds, D.