I've spent many months cycling in Italy, as well as Spain, France, Switzerland, Germany, Austria, Slovenija and the UK, and I've not experienced any difference in driving standards either between countries, or between northern and southern Italy. Most drivers are considerate and courteous, but you get the occasional knob-head wherever you go. I think your choice of road is the most important factor in determining whether the ride is stressful or not: the important thing is to plan your route so that you avoid the busier roads.
You don't say when you are planning on going. I have been in Puglia in April when it was warmer in London. Best times are April-June and September-October. Winter is iffy: temperatures are mild but it can be very rainy (ie pretty much the same as anywhere around the Mediterranean). There isn't actually a huge difference between the weather in northern and southern Italy - the weather in Italy often seems to be either determined by weather systems from northern Europe or weather systems from Africa.
My impression is that English is pretty widely spoken, at least in places that are even vaguely on the tourist map. I say 'impression' because as I speak Italian I'm not the best person to judge.
There's a route on my site that you might be interested in:
http://italy-cycling-guide.info/cycleways-cycle-routes/coastal-rides/puglia-tour/
Sardegna is also really nice. I was there this summer but haven't had a chance to write it up. Other possibilities are Sicilia, the coast of southern Campania or the Apennines in Abruzzo.
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