...was needed to get this 24 year old engine and engine bay looking like this, and how long was the water blaster on it...

https://www.trademe.co.nz/a/motors/cars/land-rover/listing/3579374994?bof=L95EQeAQ




My first reaction was that I'd be greasing the fan belt tensioner, maybe replacing it - but I wonder how many other bits have been compromised.

Probably steam cleaned, then had a liberal coating off silicone plastic conditioner.
It won't stay that way, once it's got hot and dust has blown in through the front.