Some early Audi's 2.5's were a 5 cyl and mostly mechanical. I think they shared the same bellhousing platform as the VAG 2.4td straight 6 used in old VAG's and Volvos which in itself could be another option.

Cant see any of them being overly cheap to buy though.

Isnt there an oldish Merc diesel option ?

Or maybe a Nissan of some sort ? Taxi etc ?

Ive a complete running 940td auto here sitting parked up. Probably work out way too much to buy and take home though