First of all he looks very cute.
I do understand you want a trial, but I do understand also why other party doesn't want to go with trial: they don't know you either. :) What you COULD do though is suggest them to write them on paper what they say about the horse and that you'll return him for full refund in, say, 2 weeks, if something goes wrong (like he's bucking or rearing or such). Also if you decide on him DO the exam before paying the price.
To see whether the horse is drugged (and I was told about it by the local horse dealer, btw) is 1) check any blood on his neck (although that's hard), 2) when the gelding is drugged his male private part is very relaxed and somewhat dropped, so with geldings you usually can say whether something is suspicious (with mares it's much harder). Of course, they could give him oral supplements, but personally I didn't find the oral ones all that helpful (at least to the point of making them very calm). Also (yes, I know it's far) you could of show up there without the advance note (or say you'll come at 2 pm and come at noon instead) to see how he behaves in field and while grooming/ tacking.