Check unladen weight definately as some are too heavy to carry 3 horses legally. Think you looking at a 7.5t at least.
I dont think age is too important really but get it checked by a mechanic unless you know what you are looking for.
Light ramp, floor condition, battery immobiliser/isolator, tilt cab if poss but may need a luton if you want to sleep in it. Cut through? Head height and decent width if large horses. Mmmm cant think of anything else at the mo.
A 3.5t wont take 3 horses though and wont have the day living you want, especially to sleep in. Ive looked into changing my 7.5t for a 3.5t and I would only be able to carry one horse legally. They normally have about 1.2t payload which is 2 ponies with kit or one large horse.
We have a Leyland Terrior (I think!) which is old, and amazing! It takes 2 biggish (16.2) horses comfortably, and we have once squished 2 horses and a small pony (13hh) on but never again! We've spent the last 4 days completely rebuilding it- it's made of wood, so we can do this and it looks amazing!
For the Champs, we hired a 3.5tonne and honestly, we couldn't have had more than Ellie on board because we were so close to the payload.
To carry 3 horses in a 7.5 ton lorry & remain in weight you will probably only have a tack area & a horse area. You will not have sufficient payload to have accomodation with toilet & sleeping with the vast majority of boxes.
I have a 7.5 ton Tristar box which are one of the lightest made, it has simple living, sleeping 3/4, hob & grille, cupboards & a couple of lockers & this weighs in at 5.6 tons leaving me 1.9 tons of payload. I can carry my 2 horses, driver, 2 passengers, tack water fuel etc & remain easily in weight. I may even be able to carry a pony as well but certainly not 3 horses.
You may see 7.5 ton lorrys, with living arriving at shows with 3 horses on board but I assure you there are very few that will be within their weight limit. Most will have been advertised as 'stalled for 3' (i.e. 3 ponies). In reality if you want to legally & safely carry 3 horses & have living you need to go hgv.
As for 3.5 ton boxes, they have about 1.2 ton payload. Two horses will weigh around 1100+ kg, then you have driver, passengers, water, fuel & tack to take into account. In reality they are really one horse or 2 pony.
if you want to carry 2 big horses and have living i would say a 6t or more would be most suitable.
you can get 3.5t boxes with more than 1.2t payload but thats because living/extras have been taken out...
i am happy to take 1t of horse in mine and be within weight limits(have had it weighed so i know the limits!) luckily my 3 are all around the 400-600kg mark so i can take more than one.