its very possible that you have 2 different problems and one of which (probably the spark plugs) exacerbated the other. I recently had a bit of rain water get in to my engine compartment and it soaked one of the plugs which happened to be craptastic Bosch +4's and the engine shake rattled and rolled and wouldnt hardly move till I cleaned it all up. I'd ask your mechanic what he "overhauled" the engine with. From there we can make a more speculative guess to what your issue is.
As already mentioned though, VANOS seals on the M50/52 motors go and when they do its like having a paint can in the valve cover, you can find plenty of videos on what it sounds like, which it seems you did already. For the average DIYer the process seems to be achievable but if wrenching in your engine isnt your thing, find a reputable Indie mechanic that works on european cars to give it a second look, rather than the guy that suggested dumping goop in the engine will make the sound that wasnt there before it touched it, go away.