"How difficult is it to remove and install a tailamp assembly?"

Not difficult at all. Disconnect the harness and remove the nuts from the mounting studs (probably four of 'em). The bad news is that sometimes the nuts are rusted to the studs, and if you twist hard they'll snap.

Use a good penetrating oil, such as PBlaster or Kroil (**NOT** that crappy WD-40). Usually the penetrating oil works best if you can let it soak for several days...

"...fixing the fuel gauge...It is completely inoperative."

A common problem is the nuts on the studs that mount the gauge to the instrument cluster PCB. After many years and many miles and many potholes, the nuts loosen. Because those studs also act as the electrical connections, the gauge becomes electrically disconnected.

Same for the temp gauge.

With modest work, a schematic, and an ohmmeter, you can determine if the sender works and if the gauge works.

"Is the 300 mile mark typical for a 200 series..."

It was for my last six-eight 240s and my current three 240s (my 245T is a bit more thirsty and much more dependent on the lead in my foot).

"Engine purred wonderfully, and the trans shifted impeccably. Where else am I going to find a car in the $4-600 bracket that will still run this great at 233K?"

You made out like a bandit. Be happy investing another $500-$1000 in the car and having a comfortable, safe, and reliable car for $1500 that will last you another 5-10 years and 100k miles.

Follow all the prudent advice about:
1) Regular maintenance.
2) Resoldering all the relays BEFORE thay let you down (and they all will).
3) Preemptive repairs -- fix it before it causes trouble (e.g., the tranny cooling lines, flame trap, retaining strap across the "big plug," etc...).

--
Don Foster (near Cape Cod, MA)