Hi
usually if you at the top of the sill, nearest to the rear, by the wheel arch, it will come unclipped there quite easily and you can look in, from what i've seen, when they go there worst at the back, also if you bang them with your hand, from the bottom, if it sounds like there's a load of loose stuff in there, then you need a closer look.
my car a 2004 S-type R, the sills still look like new, but when I purchased mine , the garage let me put it on the ramp and even gave me a screw driver so i removed some of the plastic clips and had a proper look, I liked the car and told him if the sills are good its mine.
as for the rear cross member as far as I know there no issues with 2002 onwards, it was pre face lift that had a different rear sub frame and was prone to rotting, these are quite hard to find now good second hand, like I say laters ones are better, don't rust and second hand ones are plenty
take it for a good test drive as well, if its a auto make sure its all smooth, most common issue is lurching and surging in the 1-2, 2-3, 3-2,2-1, listen out for any knocking or bumping, there's a large amount of bushes on these that can fail, should be silky smooth
also every time you stop try the parking brake, these also can be a common fault, you get EPB fault,
check every thing works, especially all the electrics, check both remotes work
check inside the boot, some were prone with filling with water
check the headlights, its a common fault that headlight adjuster break inside the headlight, which usually means the beams are very low, sagging, auto adjuster wont work, give very poor lighting, they can be repaired, but very time consuming, HID headlights means headlight off and bumper off to replace bulbs and to repair adjuster inside.
service history will be a good indicator on the car, more the better, look at the tyre's, descent branded tyres on the rear are a must,
these are just a few, but check them sill's
cheers
Joe