It depends how competant you are and how far you want to go, every time i've took the head off a car i've replaced the water pump, belts and tensioner as a matter of course but this doesn't always need to be done you can just stick new belts on if your budget is as tight as a nuns chuff.
Personally i'd take the head off myself and drop it off at a decent machine shop and get them to do the work, you may aswell buy a full top end gasket set which will come with the valve stem seals and you may aswell get it skimmed for good measure, machine shop should charge you around £100 for a skim and replace the seals/stirp and rebuild the head ready for you to fit, and get them to replace any worn valves while they are at it which will be around a tenner each (usually one or two are worn), apart from that i recon about £100 for a gasket set then your belts and what ever else you want to do...
And always use brand new head bolts, if you re-use old ones and one snaps you will kick your self!
