The altenator is a pretty easy change unless going from a 120amp to a 150amp as you should have a software update done (by Volvo) to tell the car it now has a 150 attached.
A good search will get a Genuine Volvo/Ford altenator for approx £150 and then any garage can fit for 30min labour = <£200 and certainly no more than £250 should be paid.
A couple of Links for those looking
1.8 Petrol S40/V50/C30
1.6 / 2.0 Diesel
