For the moss ball, just remove it from the water in the petstore, rinse it in clean water, squeeze it out, and place it in clean water. QT it for a week or two before putting it in with your fish.
Looks like that poor guy was either born with a deformity, or got injured and it healed funky. I wouldn't buy him, I'm against buying sick or injured animals from the petsotres, it's just encouraging them to do the same things. Also, I'm not sure how great his quality of life would be... If you insist on getting him, I'd bring him to an employee and ask to have him for free since he might not make it anyways. However, if you already struggle to keep up with your water changes, please don't get a new fish. A one gallon would require multiple water changes a week, so that's quite a bit of work. I know it's hard seeing animals in such bad shape, but we can't help them all. Hopefully education will break into pet stores eventually, and we won't have to see things like this!