Buying a newer car is not neccessarily going to be any more refined, that really depend's on what car he want's or is thinking about. I have spent some time driving a clio diesel of a similar age and had no complaints about it's refinement at a cruise. OK it, like most diesel's, can be a little noisy under acceleration, but once up to speed, it is fine. The fact that it has done 150k is also nothing to worry about, as long as it has been serviced regular and had things done when needed, there is no reason it couldn't manage at least another 50k.

Agreed, we absolutely love the little Clio and nothing could be more economic, however it's done 150k and he's 200 miles away so was looking for something a little more refined

If he's owned the Clio for a while, its a known quantity, including the level of maintenance he's had carried out on it. Why not save the large outlay on a much newer car and get some worn parts on the Clio replaced so it runs better. If he has scrimped on the maintenance on the existing car, he almost certainly would on the next one, given he'll have less money in the bank after paying for it.

Best to keep the Clio running witha bit of extra TLC (far cheaper than spending £8- £12k on a replacement [using a car of similar size]) for a couple of years until he gets a job, settles down in it, earns a nice bit of cash (not spending quite so much on nights out as most young blokes do), and saves up for a nice new or nearly new car as well as decent maintenance.

If an employer expects an employee to do a lot of business miles, then they should by rights give them a company car. I suspect as its only a summer job (even if more than one summer over the duration of the dgree course) they won't be requiring him to do much company mileage in it (just from home to work and back) and would normally be accompanying a more senior colleague on trips out, in their car, not his. That's what happened with me as a junior engineer nigh on 20 years ago - I 'got away' with driving my (at the time) little 1.0 petrol Micra around, no problem.

Admitedly your son's Clio has done a large amount of mileage in its life, but as a diesel-engined car built when DPFs and other three-lettered acronymn devices weren't around yet, they were still quite reliable. Unless its been going wrong a lot of late (lots of expensive parts failing due to age rather tha standard wear-and-tear items like clutch, brakes and tyres) or corrosion is bad (especially on structural load-bearing areas), then you may find, with a bit of TLC it can live for a reasonable while longer without too much problem or outlay. It would be different if he'd only owned it for a year or two and knew very little (other than via the MOT search) of its history.