Bunnies can definitely be just fine on their own if they get a lot of attention from their humans. If you want to get a second, make sure you really want one and don’t get it just for her, because it’s always possible that they won’t bond. Bonding can be really stressful and take a long time, so it’s a commitment. With that said, it’s really fun to have two and watch them interact.
If you do decide to get another, definitely look at already fixed rabbits in a shelter and let your bun meet them. Usually female/female pairs are the hardest, so I would focus on males, but it’s possible that she would get along with a female. The breed doesn’t matter at all, only the personality. Some bunnies are bossy while some are laid back, so getting the right blend of personalities is important. You often can’t tell how they will act around others until they are introduced – my boy is fairly bossy with us, but extremely laid back with other rabbits. My female is very sweet with us, but super bossy with other rabbits.
The size of her pen sounds perfect for a single rabbit. A pair would definitely enjoy a bigger space, but you could probably make it work in that size if they got a lot of free time to exercise.