A spayed female would be best, but a neutered male could also work. The age does not matter, I have a 5.5 year old male who is bonded with a 1 year old female, but has been bonded to 2 other females (at different times) who have been older. You might want to consider getting one about the same age, but don't rule out ones that are older. I would avoid ones that are much younger as they would probably not be fixed and could change with the hormones.
The rabbit should be fixed. While it is possible to one intact rabbits, it can be rough for them as one will want to mate and the other doesn't.
Bonding can take a while. You need to be 100% confident that they will get along before you put them in the cage. While they will need to decide who is the boss bunny, there should not be any fighting. Some chasing and humping is normal, but it should not be all the time.
It is better if you can let him choose who he wants to be with. Many shelters will let you take him in for 'dates' so he can meet other rabbits. Hopefully the volunteers know about bonding and can tell if it could work or will not. If you just bring in a rabbit, there is a chance that they will not get along.
I have not had issues with my bonded rabbits. There can be a bit of humping and chasing (usually after the humping as one it not too impressed with it), but there is no aggression. My angoras do not get along at all and there have been some fights, so I have to make sure they are kept separate and the cage is secure.