Type of box is fine- but it is cheap, possibly for a reason? If its any good, it will have probably gone already, but if you do go and view it, take a mechanic.
I looked long and hard for a box 20 months or more ago- and ended up paying more than I wanted(had to get a loan) but had a new build done for me as the older boxes tended to either be sold befoe I got there if they were good, or had problems. It cost me alot getting a mechanic to check them out, but in the long run saved me more by my not buying a pile of junk.
If you have time, trawl through my posts last year about purchasing one of these boxes, as I learnt alot in my efforts to get one. Buy the best you can afford, as you are travelling a very precious cargo in it!
I have not long bought a box of this type (although mine is a conversion rather than a coachbuilt) and I love it. Bags of room for my 15.3 section D and super easy to load and unload a horse on your own. You just have to be very careful about the weight if you plan to travel 2 horses as it is easy to go over the 3.5t limit and become illegal.
These small boxes have had a bit of bad press lately, but I don't agree that they are really any more dangerous than other types of horse transport, as long as they are well made and well maintained. I have seen horses do very stupid things in both conventional lorries and trailers. I do think that the ones with a wide or double rear door (+/- removable breast bar) are better though, as you have an alternative exit for the horse in an emergency situation.
I looked at a fair few older ones like the one in the advert, and in the end I ended up borrowing some money and having a new conversion done on an 05 chassis. Mine cost me £13,000 altogether (including fancy extras like a posh paint job and camera system). What influences the final cost is the cost of the chassis, so you may get better value for money and a better vehicle by having a conversion done. Having looked at a lot of older ones, you pay a lot for the fact that its a horsebox when actually you are buying a very old chassis that could be prone to a lot of problems, they are overpriced really if you compare the price guide for the commercial vehicle equivalent. Also you don't know the quality of the conversion - there are some real cowboys out there producing stuff that is not necessarily robust enough for a horse!
A G reg vehicle is about 1989 reg if my memory serves me correctly, so that is a pretty old chassis for £4.5k.
Just to add - go and have a look, but take a knowledgeable friend/ tame mechanic with you to give it a once over. I have heard that Mk1 Masters are bad for rusting. Newer ones have an anti corrosion coating on the chassis I believe.