Oil filter bracket probly just needs tightening (8mm allens?) not a new gasket. And if it was me I'd cut the A/C belt off, replace the air filter housing mounts and just start driving it everyday - then draw your own conclusions regarding engine performance/longevity.

Previous owner sounds like he gave up on the car and couldnt figure anything out. His "dead engine" diagnosis sounds like bunk. Nice score snagging an excellent 4-spd 240D that will probly run another 100k miles at least. My guess is it's the engine mounts that brought down the rest, not a dead engine at all. Meanwhile decent running 240D engines with less then 250k miles will soon be getting scarce. Are you sure you want to swap engines and throw away a perfectly good running 616?

(edit: And its interesting the PO replaced cosmetic door seals while neglecting the engine compartment.