Replacing a PSU is easy. The key bit is above: make sure it's rated high enough. I keep a spare in the cupboard - a 500 watt.
From experience, repairing PSU fans is a loser. The bearings usually go before the blades do If you can get the right fan, replacing may work, but on the whole a new PSU is a lot easier.
Ebay seems to offer a good selection. I fixed one with a PC World £50 jobbie for a friend who was in a hurry, and the new one lasted only about 15 months. We weren't impressed.