A second child wouold definitly put more stress on your marriage. Can I suggest a date night each month or each week? We have a 4, 2, and 8 month old. We have no family that is available to sit for us. But last week we started doing datenights again. We have the kids all ready in their jammies. They watch a short video, while the sitter puts the baby down (with pumped breastmilk), and then the sitter puts the big kids to bed. We went out from 7-10. This week we'll try 6:30-9:30 and see how that goes. It is $20 a week, but worth the investment. You don't even need to spend a lot of money when you go out. Just go for a walk, to a coffe shop, or browse a store. At home there are too many distractions. And in public I doubt your husband would do the gross stuff, like pick his nose :)
Good luck! Take some time to nutrue your marriage now. You can always consider adoption once your marriage is on a better place. That way the age and health factors aren't so much an issue, and you can get an older child (1-2). who may sleep through the night, ect :)