Just look for a good one at sensible money. Everyone wants a diesel these days it seems, I like them but they do cost a fair bit and aren't as reliable as some of the petrol engines out there. M112 is pretty much bulletproof. Would definitely rather this than an M271 or a CDI engine - both of which can be expensive when they go wrong.
I changed the crankshaft pulley, belt tensioner and idler pulley wheel as a precaution on mine. Tensioner/idler were very easy DIY, had the crank pulley done at the garage. Not pretty if they go apparently.
Had the autobox & filter changed as well.
No rust on mine other than a few scabs on the front wings which I just rubbed down and hammerited
So far so good.