250 miles is not the same distance all over the world LOL.
OK so I know that the distance is the same, but the type of journey and the length of time it will take are completely different.
I'm guessing that the OP isn't in the US, and also by the use of 'lorry' I'd also guess that they are in the UK, where 250 miles is a loong trip.
Depending on the time of year you want to consider what time of day to set out, if possible avoid the heat of the day. If you are in the UK intense heat may not be an issue, but I may still consider an overnight journey, the roads will be a lot quieter and there will be less start and stop traffic.
Have you hauled these guys before? If not start practicing loading NOW, there is nothing worse than having all your plans wrecked by a pony who wont load, so get that issue dealt with ahead of time.
Think about your loading order, I like to make sure that you put friends next to each other if possible, but even more important is to avoid putting two horses who hate each other together, even if there is a partition they will still argue. On the same lines I always want the first one to be loaded to be a 100% easy loader, I want the others to see 'Fred' just walk up the ramp with no issues.
I'm guessing that you will be using a slant or an abreast loading system, so tie them up with heads to the driver side of the vehicle, that puts more weight towards the center, and therefore flatter part of the road.
Different peple will tell you different things about 'dressing' your horse for travel, boots, no boots, bandages, no bandages, my personal choice...if at all possible leather headcollars, nylon as second best. Unless you are skilled at bandaging, or have some help then I think that you can actually cause more problems than you solve, so I usually opt not to have leg protection. I don't usually rug or blanket unless I'm traveling hot horses on very cold days. You will have to make up your mind on this one
I personally like my horses to have a hay bag, or hay net when traveling, but once again a bit of personal choice there.
For water I would have buckets and maybe a container of water on board just in case of hold ups, it's nice to be able to offer a drink if the journey gets to be longer than expected.
Of course make sure you have horse and human first aid kit on board, and spare halters/headcollars/ropes, just in case.
If you want to be super safe, once you have your route planned you can look up the numbers of vets on your route, just in case emergencies happen.
Most journeys go off without a hitch, remember the horses are not worried about it at all, they will only worry of you so, so relax and enjoy the experience.