It is always better to replace everything, Including the water pump if it is driven by the belt or if it would involve removing the timing belt to replace it at a later date.

When you dont change a water pump thats driven by a cambelt you expose the old water pump bearings to new loads from a new belt and tensioner that from experience leads to its failure within a few months of the old WP and a repeat of all the work. Customers attending my garage for a timing belt replacement on a VAG diesel for example where the water pump is driven by the cambelt are advised to have the WP changed as well with the additional cost of new WP and antifreeze shown. Those who decline are invited to sign a disclaimer to the garage for not changing the above. If they wont sign the disclaimer I wont do the job. Some of the GATES cambelt kits now are supplied with a WP as part of the change (where applicable). Note not all WP's are cambelt driven.

If a customer can see I'm prepared to turn the work away from my workshop because they wont sign the disclaimer for doing the job on the cheap most accept I'm not having their leg and have the job done properly. this recoomendation comes from GATES the OEM for most vehicle manufacturers cambelts and the "designer" of the belts for an engine and not the manufacturer or the "dealer" who just follows the bulletins from his manufacturer.

On the recent GATES course we attended they have designed most timing belts today with an HNS construction to last 300000 Km or he life of the car. So its not about them wanting to sell more product. They guarantee engine replacement for a failed belt provided it has been fitted correctly as a kit. They analyse a belt failure on a claim and can tell if the belt was over tensioned or otherwise on installation or indeed if it has been removed and refitted. We fit these belts as a guarantee to the garage that should they fail and we have fitted the replacement correctly the garage is covered for an expensive engine rebuild or replacement. (Old story many still have to learn "you get what you pay for")

Hope this helps

(aside not recommended)

Incidently we have a customer of ours who tools around in an 03 plate Lexus 430 with 360,000 miles on the clock who decided to get his timing belt change (he'd owned the car from new). We reckon the old belt had about 100 miles left in it. (Talk about lucky) !!