When you go look at him, make sure that you ride him and handle him on the ground. A VERY thorough vet check is a must on a horse that age. When you get him home, I would put him on free choice grass/alfalfa mix hay and a good balanced senior feed for added nutrients and maybe a joint supplement as a preventative. Nowadays, 25 really isn't that old and horses are living to be older and older all the time. There are some considerations to take when owning a horse that is "elderly"; their body heat regulator doesn't work as well as younger horse's so they get hot faster and it takes longer for them to cool off. There is more risk for health problems cropping up, they sometimes are harder to keep weight on. However, all that can be managed with proper care, handling, and feed.
As for what you should do with him the first few days; if he was mine, I would probably ride him that first day but a lot of people don't agree with that. You could do some hand walking/grazing, working with him on ground manners, messing with his feet, grooming him, maybe a small amount of lunging if you want. Just spend time with him and try to connect. I would watch his reaction to the new surroundings and depending on how he reacted, I would advise you to wait anywhere from a couple of days to around a week before riding him the first time.
Do you have a way to keep him separated from the rest of the horses but where he would still have access to them, like an adjoining paddock, or could you put a pen inside the area where the others are? That way, he could get to know them but still has a security curtain to protect him from them. If not, you could always just take him out and turn him loose with them and keep a very close watch for the first couple of hours and during feeding time.