When you purchase a new horse, try to keep him on the same feed, at least initially, until you can wean him over to a different feed if you want to. This includes hay, as the hay you have will be different than the hay the horse was eating. Although all the hay may be coastal or alfalfa, all batches of coastal and alfalfa are not the same. Then there is the concern about the pasture, as your pasture is certainly not going to be the same as the one the horse was in previously. So it is important to also get some hay from the previous owner and gradually wean the horse off onto the new hay and gradually introduce your new horse to the new pasture by limiting turnout. It is important to take the horse on a lead and walk around the entire pasture and slowly acclimate the new horse to your old horses. Show the horse the water source.
It is not a bad idea to keep the horse up a few days to accomplish these tasks. The reason this is critical is the microorganisms in the horse's hindgut require gradual changes because a quick change in any feed upsets the beneficial microbes and can lead to colic or laminitis and founder. Most horses require a gradual transition period of two weeks to change feeds so the microorganisms can adapt. If you cannot get any feed the horse was being given, then gradually start adding in the new feed in low amounts and build up the total amount over two weeks. It is also important to feed small amounts frequently instead of just once or twice a day. Introducing a new horse to new feed may take some extra time, but may also save you from having to treat a sick horse.