The turbo(s) are one of the more reliable components on a Volvo.
The turbo's are more reliable then the electronics, transmission,
and air conditioning (and many other parts). So, you don't really
need to worry about the turbo any more then any other part (and maybe
even less).

Compared to a non-turbo the cost of ownership will be very close
with variations from car-to-car being more important then turbo
vs. non-turbo. By car-to-car I mean some cars have more problems
then others and so it is hard to predict exact costs for a specific
car; but for the general case, a turbo and non-turbo model will
cost about the same to maintain.

And yes, synthetic oil is a very good idea with a turbo since the
oil passes through the turbo and the extra heat is handled well by
synthetics. Also, I belive synthetic oils are less likely to
"coke" and leave deposits on bearings (such as the turbo's bearings).