I think the biggest risk you will take in buying them is; a lot will depend upon how they have been stored up until now, i.e. have they dried out too much.
The chances are these are the remains of stock that has been picked over and for some reason left.
If they are in those plastic bags with holes in they might have dried out but at 50p they might be worth a go.
You mentioned growing them on then allowing them to dry out and storing them, to my mind this is your best approach at least you will find out if they are viable before going to the trouble of giving them winter storage.
It should be simple enough to see if the gladioli are viable by looking at the base of the corm. If you can see a ring of growing points (dimples) that are pale in colour as opposed to dark these could be OK.
With the orchids squeeze the pseudo bulb and it should be quite hard, if it is soft don't even consider buying.
The montbretia (crocosmia) might be the greatest risk as these don't want to be totally dry, they should be what I describe as the 'damp side of dry'
Let me say the above is only an opinion, the choice of buying or not is up to you!!
