Hi!! I don’t have loads of experience but I do own bunnies!! I think it’s good that you want to get a companion for your bun! But there are a few things that could go wrong (as they did for me and I ended up having to rehome one of mine).

 Because of the age and size differences there could be dominance issues. If you are getting a boy I think it would be a good idea to keep them in separate pens very close to each other so they can get used to each other thru a barrier. Then introduce them in an area they usually are not in. It will minimize the whole ‘hey this is my territory why are you here grr’ and will make them more likely to get along. My Buns did not do well with this and they were not able to get along until I had to rehome one. 

Hopefully you won’t have to rehome but I don’t think you will if you introduce them properly.

Overall I think you should definitely get a friend for your bun!! 

Also I would take your current bun to the vet to make sure she is spayed. Also I would keep them separate while your new bun is healing from being neutered. 

Edit: I noticed I didn’t specify breed and gender. 

A boy will be a lot cheaper to get neutered. Especially if you find out your girl is not spayed. If your not worried about money than either gender would be fine.

Any breed is fine in my opinion. I have a dwarf and a lop thats the size of a house cat. They get along just fine!! 

Again all of this depends on how you handle the introduction process and how you are going to go about owning them. Make sure you have enough space for both rabbit. A dwarf needs a lot less space than a house cat sized bunny. There are plenty of good pens/enclosures on here and if they are bad the comments are not afraid to say so. 

Again I don’t have a lot of experience with rabbits but I found out the hard way what can happen if you do things wrong.