[v8volvo]

80C thermostat will be the best choice in your climate, or any other scenario of prolonged operation in hot weather involving high speeds or hills.

For a "complete" timing belt service on this engine or any other engine like it, the rule of thumb is to replace the timing belt and any service parts it touches. For a D24 or D24T that means the timing belt, the idler roller, and the water pump. Even if the water pump is "good" for now (meaning no leaks, bearing OK), if the bearing goes bad later on within the life of your new belt, then it'll take the belt out and the rest of the engine with it. (Or if you're lucky you'll notice it first, but then in order to replace the water pump, you will have to do most of the timing belt job again.) Therefore, the logic is to replace these parts as a system so that everything remains trouble-free until the next routine service interval.

The rear timing belt, which drives the injection pump, should be replaced as well since it is removed during the front timing belt service.

As ngoma wrote, though, there's no point in buying the belt parts unless you have a plan for how you will successfully install them. The required special tools and procedures are mandatory. You WILL NOT be able to replace the timing belt successfully without them. If you try, your best case is that the attempted belt replacement will be extremely difficult and, if you manage to get the engine running again, it will start and run poorly. Other, more dire (but common!) outcomes are that you may not ever succeed in getting it running again, and/or the engine will be destroyed by a sheared crankshaft gear.

Replacing the timing belt proactively is necessary because if it breaks the engine will be ruined. BUT incorrect installation of the belt can be just as destructive.

Those are the warnings. But here is the good news: with the correct tools and process, replacing the timing belt is a straightforward job and you won't have to do it again for many years.

The tools are specialized but not that hard to acquire. Volvo dealers still sell them, or other owners may have sets they will share. Here on this forum, members often loan tools to each other for this work. Many also have had good luck borrowing or buying these tools from independent Volvo specialist garages, or from Audi or VW specialists who deal with the 5cyl version, or on ebay.

As for the procedures, the official Volvo instructions for replacing the belt and associated components can be found in the "greenbook" repair manuals which are available for free online in various places. If you can't find one, post back and folks will point the way. There are also instructions and tips posted throughout the forum here that can serve as an alternative or an addition to the greenbook procedure. Any questions you have that you can't find by searching, just start a thread and ask.