The rebuild at 110k is not that unusual, perhaps it was done by an owner who wanted his car running 100% tip top, perhaps the "rebuild" was a bearing replacement or something....and I would rather have an engine with the work already done than one needing it done and the seller is trying to offload it for me to have to pay.
The key to buying any car is to look at the maintenance history of the car, has it has all of its services carried out, has preventative maintenance been done or did things have to break before they were replaced, has decent oil been used, did the previous owners put decent tyres on it etc etc. All these things give you a good idea wether the car has been cherished or used and abused.
There is no reason to suspect a high mileage gearbox is going to fail but of course the older/more used it gets then the odds increase that something will break but bear in mind that cars only a few years old with massive mileages have seen motorway miles and not much start/stop town driving so they havent been stressed much.
Take a look at some BMW forums and you will see people with 55 grand cars moaning about warranty claims, repairs, huge bills for faulty engines etc etc etc and you will see that Alfa's are generally no less reliable than any other car of its class.
My advice is, if you are serious about the car after considering the above, have it booked into a reputable specialist who can give you an honest opinion after looking it over for an hour or so. The £50-£100 this will cost you is a small price to pay considering you could be investing your hard earned cash on what could either give you either years of pure enjoyment or years of misery and an overdraft the size of Michael Jacksons solicitors bill.
The bottom line is that owning a 24v 3 litre V6 Alfa is NEVER going to be cheap once it is out of its warranty period, anyone who thinks otherwise kidding themselves or they may have been lucky. You buy a car like that because you want it and because they are so damned good to sit in and drive not because it is a sensible investment
ps: I hope you like auto boxes...personally I think that engine deserves a manual... try flooring the throttle in 2nd and 3rd gears in a 5 speed and you won't ever want another car.