I have a Renault master and absolutely love it. If I'm not mistaken the Renault and vauxhall are exactly the same other than the badge, built on same chassis etc. I think too Renault master come in bigger engines I have a 2.8 turbo diesel. Best thing I've ever bought
Remember that these almost identical vans were designed for a six year working life(but worked damn hard) .Their servicing is designed to be fairly in depth and per mile is not particularly expensive. On a low mileage per annum horse box ,this can prove expensive.Also look well for rust . I recommend that you have the underneath steam cleaned and then treated with waxoyl to reduce corrosion.There are a lot of nooks and crannies for rust to develop.As said ,they were designed to last six years.
I have a box from these after many good reports about their boxes. Also looked elsewhere and found these to be the safest horse wise ( they have an emergency exit through the back doors incase of accident or horse going over the breastbar which many other conversions do not have) and one of the lightest too. I part-exd my 7.5 Iveco box for one and must say that we have never looked back. You can choose your own colours and vinyls etc. Mine is a master but they will convert either and you can either source your own van or they will find you one that fits with your budget. They are really nice people too with their own horses so really pleasant to deal with. If you want OP I could send you some pics of mine or feel free to message me with any questions