I have learnt loads since looking for a box
Check the paint work of the whole box - if it has had a very new re-spray without anything else being done this raises alarm bells. A poor re-spray will cost £500 tops, and will cover a multitude of sins to the majority of people. Look at any edges/joins - any bubbling, rough or uneven patches suggests job wasn't done properly (should have been sanded down and treated) and will need doing again. Open doors and check the sills as if a poor job they won't have bothered with doing them
Chassis - Be wary of ex-delivery/transport vehicles as they cover very high miles in a very short space of time. Some companies shed their stock after 180,000km which in 7.5t wagons is nothing and will be in good nick. All cabs will go round the lower bit of the doors, the door sill, footwells and certainly on Ivecos will go at the front (lift the bonnet - they go rotten around the bonnet hinges and on that support bar, and will also go on the engine mountings and there is two areas at about waist height (don't know what they're called) that if rotten is not a good sign
). Milometer should show level numbers (apart from the last 2, possibly 3 numbers), if all the numbers are different heights (and you're not on the cusp of changing .i.e 299,999 to 300,000) then be wary as well
Lift the rubber/plastic seals and flooring in the cab and check the metal underneath. Discolouration is fine, but rust (spotting tolerable) is not. MAN cabs seem to be predominantly plastic in the these areas so not normally an issue. Feel under wheel arch, if the surface feels flaky and if tapped feels 'tinny' then have a closer look as this'll mean the floor pan needs attention. If it looks new under there, take up the floor in the cab to check it hasn't just been plated over. Even with wax-oiling you can still see erosion. You can clean the cab inside to within an inch of it's life, but you do struggle to get rid of the orange hue of rust, so if a cab doesn'y have an orange hue on the floor and no rust spotting round the pedals and gear stick then you're probably okay
Take any new shiny hubcaps off and check wheels. Poor looking wheels can often lead to poor suspension and chassis. Ask how long the box has sat as sometimes the suspension/brakes etc. just give out on the first time home because they haven't been used. Boxes stored indoors will be less susceptable to this.
As a general rule, if the cab is okay then the rest of the chassis should be okay
Box area - if it is a granulistic surface (looks like tarmac) then you'll need to look underneath. This is not an ideal surface as it can mask weakness in the floor and is a b*gger to replace if it wears out. Wood can be worn, but not have any staining in any one patch (which would suggest wee ingress from above). Be wary of untreated brand new wood as in my experience this has meant other short cuts. Checker plater is only okay of it covers the whole of the floor in one/two sheets and is secured round the edge and is not the 12" square tiles. Don't be afraid to lift up the rubber matting (if laid), and if screwed down, ask the owner to lift a corner for you. Have a jump round the box as well to check for soft points and see what the suspension does. Air suspension will act differently to other types of suspension.
With the ramp down look at the lip (bewteen the ramp and box), this should ideally be clean and free from erosion (slight rust colour not a huge issue assuming it's smooth - any bubbles or rough spots and I'd be checking the underneath of the floor and all the metal work).
Ramp should be easy to use, shouldn't creak or feel as if it is 'dry' (feels as is it needs a good dose of WD40). It shouldn't land on you, but needs to go up easily without needing a 7ft wonder to push it in at the top
Ramp shouldn't be warped, or be seperating at the edges, hinges should be free of erosion.
Internal walls should be solid feeling, any fixtures like wall matting you'd expect to see attached either flush with no metal work or with mushroom caps and washers (smoother finish). Metal work should be smooth and have not sharp edges.
Gut feeling has alot to do with it as well. Sometimes things don't feel quite right and 90% of the time it'll prove you right
Oh, and talking to several machanics and motor engineers Ford Cargo/Ivecos are preferable. Newer Dafs are bearable but finding parts for old and new can be awkward, Mercs okay and solidly built, but not great suspension for horses, MAns again are solidly built but not common and renaults apparently are a no-no unless HGV.
Same principles for smaller boxes, but a proportion element comes into it. Boxes that are higher than they are wider (like box vans) can be twitchy when weighted and feel unstable. Boxes like the Sonic are better as have a lower floor so have a lower box and centre of gravity making a more stable ride.
Check your net weights of the boxes as well, especialy on ones with living as sometimes they state you can fit 3 x 17hh horses in (which you probably can), but you'd be over weight in total.
Sorry for rambling
but I hope it helps!
