Hi
The 2.5 and 3.0 v6 dont have a cambelt, they are chain driven, so as long as milage is low and there quiet, will last, high milage cars, timing chain can rattle when they stretch with age
the s-type diesel are of tdv6 2.7, v6 twin turbo, generally reliable if well looked after, later DPF models can give more problems, these have two belts which need replacing, cam belt and fuel pump belt
check every thing work, all the electrics, take it for a good run, check the 6 speed auto box, suspension should be silky smooth, any knock or bangs could be a bush, there's a lot of them on the s-type
check for water in the boot, it leaks in above the rear lights, use something like vehicle smart to check cars mot history
one major check has to be the sills, some suffer more than other, not down to year, later ones also suffer, the sill can completely rust away, which are hidden by plastic covers
cheers
Joe