cant beat M52 for bang for buck, reliability, good power upgrade, light weight, easy swap, a million people done it so plenty of data on the internet what works and what doesnt. i can bang a conversion out in a weekend now once you have all the bits.
Really comes down to budget, and i defiantley run with the school of thought while the motor (regardless of waht you put in) is out, change all the gaskets, seals, o rings, spark plugs, etc. If not everything then at the very least especially the stuff on the bottom like sump gaskets and rear main seal, timing cover that are hard to get to once the motor is in (again, regardless of the engine you choose. makes it a lot more enjoyable once finished if everything works as it should, doesnt leave a mess on your freinds driveways, etc.
$5k will get you a fully certified M52/ZF swap including all the other stuff to think about