Oil leak (fragments in oil)

1 year ago I bought a 2006 BMW 3 series (2l Petrol). Other than having to replace a coil, there have been no issues up until a few days ago.

It became apparent that oil was leaking quite heavily from the oil filter housing. The garage confirmed the oil levels were very low (1.5 l) and more importantly they told me small fragments of plastic or metal were found in the oil?!

The garage has since replaced the seals to stop the leak and added oil. What worries me is that they can't tell me how serious the problem is? They said this can only be done by disassembling the entire engine which is an expensive job. They speculated the fragments could originate from the timing belt. They said in some cases I would be better off getting a replacement 2nd hand engine??

I don't know what to do. The car sounded awful before and now it sounds better, but I have a feeling something is not right.

I have two questions.. 1. Can anyone make an educated guess given the limited information available? 2. Is there a diagnostics service out there which could assess the damage accurately?