Depends on how old the filters are, how much water he produced using it, the ingoing tds...all of which will give you an approx to how long the filters will last for if you buy it...
Imo £300 is a bit too high...especially if you're not producing alot of water as merlins are built for high water production
Also if you're on a water metre then they use ALOT more water than a standard r.o
If you not going to be using alot of water, my suggestion would be buy a new 450gpd r.o for £170ish and a d.i second hand and produce into a holding tank (you can pick up an ibc cheap as chips)