I have one on 54k which I don't consider to be particularly high, but I went for a car purely based on condition, history and the way it drove.
I first drove a manual 04 plate with a manual box (about 45k miles) in Dark Blue, it very nearly put me off the 4200 all together. The box wasn't as hard as people mentioned but the whole car just felt lazy despite recent spark plugs. (I saw that one bought by a dealer, bits of paintwork fixed and then sold).
I then jumped into the Cambiocorsa I have now and it was like night and day, it felt considerably perkier, everything works and is in lovely unmarked condition. The service history shows packs full of specialist receipts, recent'ish clutch (less than 8k miles ) and it hasn't caused me any issues (in fact slightly less troublesome than my Alfa GT which has been nearly as good as gold).
Also the later the better, most consider the 03/53 plate onwards model with the revised settings to be the preferred car (normally spotted by the V8 badges on the wings, MSP traction control button), but I think the 05 onwards brought further improvements again with a slightly different nose and used as the platform for the Gransport.
The key IMO:-
Regularity of use - I use mine at least twice a week
Service History - You can tell from the history the mentality of the owner and when you have an invoice for the most minute of screw covers/air vents due to chrome peeling e.t.c It can only be a good thing and shows a love for the car. In the same vane it really doesn't bother me and sometimes a benefit when the car has been maintained by a specialist rather than a dealer.
Engine - Very strong - regular oil changes and spark plug replacement when specified
Check all the electrics
Clutch - Probably the most expensive service item
Brakes - Alternatives available at fair money
Check the online MOT records to look for past failings/advisory (again few advisory and lots of receipts, gives an indication of care).
Check how the owner drives the car, do they drive at speed over speed bumps or take a bit more care?
TYRES - For me are one of the biggest giveaways, the amount of 4200's I saw on LingLong/WiWoo's or whatever nonsense tyres were astonishing and I would start pondering how the rest of the car has been looked after. The cars push out just short of 400hp, you need as much traction as you can in wet/dry.
In summary: They are surprisingly reliable sports/super cars, that may have the little hiccup but generally a well sourced example will give you far more smiles than pain.
Enjoy!