They may be old and cheap to buy but then can still happily hit you with the prestige marque bills.
This is not so much the case anymore: they've got to the stage where parts are plentiful enough that nothing is really that expensive to fix, bar rust. In which case, having a good look at the sills, arches, previous MOT advisories, mounting points etc can easily determine if the one you're looking at is worth having or not.
The age of car you're looking at can be DIY serviced quite easily, which is not so much the case for the newer X350 shape as they have more advanced electronics and complex codes which cannot be read by cheap eBay code readers.
Chances are, unless you're lucky enough to come across a mint example for not much money, it will require regular little jobs doing on it. From a financial perspective, this is unlikely to be a big deal, but some people resent spending time every few weekends fixing one problem after another: depends if you're one of those people or not.
You hear lots of horror stories online from people who've bought an older premium marque car, taken it to their local "specialist" (many of whom are specialists only in ripping people off) and come up with a bill for twice the value of the car. To put things in perspective, I've had two old Jags, both X350s which are supposedly a lot more expensive to run than the X300 / X308.
Car 1, used from ~110k-170k:
Only non servicing related expenses were as follows
Diagnose and repair electrical fault (turned out to be rear fuse box): £300
New heater resistor from eBay: £40
New lambda sensor: £60
As far as I can recall, that was it
Car 2, used from ~100k-currently 135k:
Suspension strut replaced by used item with 1 year warranty: £100 (would have preferred to spend £350 on a remanufactured one with 2 year warranty, but parts were out of stock at the time)
New heater resistor from Jag: £160
Heater core flush: £10 for some coolant
Air con regas: £45
I don't think that's bad at all personally. All those jobs bar the air con regas and the electrical fault diagnosis were easily DIYable, and that's on a car that's less easy to DIY than the older ones you're looking at.