This is the M54B22 - great engines! Wonder why the engine was rebuilt at such low mileage though?
There are lots of common E46 issues to work through, but by now they are so well documented. Mostly cooling system issues to worry about, make sure all the cooling system parts are in good order. There are common areas where you may see fluid leaks. On the engine look out for valve cover gasket leaks, oil filter housing gasket, rear main seal, sump etc. But probably not likely since the engine was just rebuilt. Vanos seals if worn can cause problems, DISA units can fall apart, vacuum leaks from old perished intake boots. Blocked PCV is another problem that can develop. Infrequent (and by this I mean even the normal BMW service intervals) oil changes can lead to the build up of sludge.
But if these engines are well maintained and in good running condition, they are truly superb and offer a good balance of power and economy.
I would just try and find more about why the engine was rebuilt, and who did it.