We have got the small Ivor Williams, and bought it new five years ago. It will hopefully last us as long as we need it. I have also found that for a bigger trailer I would need a bigger car, currently my 1.9TDi Skoda estate does it all, but only just (if two or three large pigs are in it...).
It is quite versatile, we can take the livestock roof off and fit side mesh for carrying high loads (15 square bales of hay fit well) and large round bales (1 at a time).
I would echo the advice and buy new if at all possible and think about versatility - ours is used a lot for carrying all the wood we need for building sheds, fences etc.
I know there are quite a few people who use horse boxes for taking to the abbattoir, so it should be fine.