Others on here are more expert on Brompton upgrade history than me, but I don't think there has been any significant changes since 2014.
incomplete list of the more relevant changes:
-2013-
spider cranks
double wall rims
brake levers
- 2015/2016-
new rear carrier
new roller wheels
-2017-
new stems for M and S models along with new bars
underbar shifters
new bell
new grips
new rubber bung
slightly different telescopic seat post
saddle with option to take a rear light
-2018-
new brake calipers
new standard tires
new rear damper
-2019-
new lights
dynamo by SP instead of Shimano
better taper system for the stem
That's more or less it. So if the bike in question does not have the 2013 changes it would not worth thinking about. If it has them I'd still consider it to be way to expensive (for UK). Advantage of a new one depends on the model - the better dynamo, the better rack, the new stem and bars, the new rollers are clearly an advantage, but not relevant on some models. The underbar-shifters are nice to look at and work well but at least the three speed one seems to have had issues from time to time (probably due to variability in production of the parts) - possibly they are gone by now and in general I clearly prefer the new shifters over the old ones. Apart from that there have been a lot of smaller, less relevant changes that still add up and generally make the newer Brompton the better bike.
In my view, the last worthwhile upgrade was improving the weedy brakes, but that was a long time ago.
It was, but the levers which are as important were only changed in 2013, so possibly not yet on the used bike in question.