Cannot compare Merc repair cos with a VW group car.
Far east cars have terrible issues with spares, new or old.
My 16 plate Octavia is on 135k even original clutch, so its cost us, aside from service items, £43 to replace an ABS sensor and £80 for an electric water pump B.
Even the cam belt change was under £400.
& diy servicing is well within the scope of a home mechanical, huge number of youtube videos out there on "how too"
We also run an older SEAT, our 2nd one, both over 100k, just routine servicing.
Spares are everywhere, Autodoc in Germany, can often under cut uk suppliers like GSF or Eurocarparts.
@BigLooser Can you just take the car to another garage? i say this because your bill so far would not be anything like enough for a replacement engine, unless they have put in a 2nd hand one .... which would be of unknown condition.
You are unlikely to have had a blown engine AND an electrical issue, so it seems to me they have, assuming they are not lying, replaced the engine for no good reason and now have figured out its an electrical issue.