Welcome to the LOC
The car looks to be in very good condition and even the alloy wheels seem to be in good nick (they have a tendency to bubble and flake) and 3K seems a very fair price to me, so it looks like you've scored a very good deal.😀
You know about the caliper issue, for the diy'ers it's a simple task so should be no problem for a garage mechanic and not expensive - the trick is finding someone reliable to do it as part of the service/maintenance. In my experience is they're not interested in doing it because they don't want the potential hassle of dealing with seized caliper pins - they'd rather just get you to pay for replacement calipers when they fail as the job is much more predictable. Calipers are not particularly expensive - I had one of my front calipers replaced earlier this year and the part cost was £110
It's a heavy car so brake wear can be on the high side but that will depend on your driving style I suppose.
Regarding the water pump, I wouldn't replace now unless there's any evidence of leaks or belt wear. The wisdom is to change the pump at around 100K miles but I would be inclined to wait until the next major service at 120K when the spark plugs should be changed. This involves some disassembly of the engine and that would be a good time to change the pump and belt as well. The pump and belt part cost is about £120
Other than that you should be good and enjoy happy many happy miles