I’ve bought an sold a few M54’s…don’t have any at the moment.

The best (and usually cheapest) used engine is one that you pull out of running parts car, with known mileage and which you can run a compression and/or a leak down test on.

Failing that, buy one from a dismantler with a warranty, but make sure you read (before you buy) and follow EVERY word of the warranty…they are written to make it hard to collect.