Renault Grand Scenic Blown Turbo

[zbc]

First up I'm in France in case it changes anything. Apparently the Turbo on my Diesel Grand Scenic has blown, thrown bits of metal into the engine and trashed that. Garage tells me it will be €20k to replace engine (and turbo). Renault are kindly offering to reimburse me 5k as a commercial gesture. I think this is happening because they have also decided to void my guarantee (which would have paid 70%+ of it) because one air filter exchange was missed on the last three services. Needless to say I'll be discussing this with the local garage who serviced it but they've been brilliant for us over the last 20 years and it seems incredibly petty of Renault to take this point but... I have a couple of other questions though before we go to the garage on Friday.

Does this seem like a reasonable repair price? The engine alone is 10k from Renault so it doesn't seem far off.
The car has 100k km and is 5 years old so it seems a little young to destroy a turbo with no warning but hey ho, stuff happens. It looks like it would be about 19k to replace with similar over here so the garage are offering to take it off my hands for an amount, to be discussed, to use on another car. As my children are grown up now I'd been thinking of replacing it anyway and as we're renovating it at the moment was thinking maybe just a cheap Dacia workhorse for trips to the dump and maybe sell this in a couple of years - any thoughts or recommendations?
Last point - is it just worth recovering it and trying to use my independent to source a motor and do the work? I'm thinking there's just a pile of pain to come in the next few years anyway with a car at this age.