Wall hung toilet on a metal support frame as most are?
Unless its a Geberit then there will be alternative parts that could be used but it sounds like you have a fitter rather than a plumber.
Worst case would be to buy a suitable replacement cistern, the flush valves all work on the same sizes of pipe, two actually so most are produced to convert.
If yours has a remote push button flush the these are also available as universal replacements for €25-35. Fitting is straight forward in most cases as they all work the same way, except Geberit.
Without photos it is tricky to see what the problem is but it certainly isnt the kind of money you are being quoted. I am back in the UK now but otherwise I would have fixed it for half what you were quoted including the long journey cost!