I have an alfa romeo, only for a month though so I can't comment too much on reliability. From what I've read they seem to have sorted out a lot of there reliability problems but I think they are still notorious for electrical problems. They are not the worst though, plenty of marques have electrical problems these days due to the increased complexity of the electronics.
My friend's parents have a 147 and have had no problems with it in 3/4 years so the 147 maybe an improvement.
I would be tempted to say buy from a dealer as a warranty is always a good safety net, but if you know what to look for then there is no reason not to go private.