Adopting a 4 yr old 50lbs “mix” what should I expect?

I have 2 cats and we live in a rather large 2 floor apartment. We have the ability and time to walk him a lot, but he has a minor reactivity towards other dogs in the kennel. How worried should I be? I want him to be okay with the cats. Current plan is to separate the cats and swap scents for at least a week (since we have two floors this is very easy) but I want to make sure I am 100% prepared for this decision. I have owned a dog before, but I was living on a farm then and had significantly more space. Is this enough space, should I be more worried about the cats and dog getting along? Any advise would be great, I want to give my pets all the best care I can!