I would really really urge you to get another female. Rats are VERY social and no matter how much time you spend with them you cannot take the place of another rat, period. You cannot groom them, play with them the way other rats do, be there for them when they are sick in the way they need and so on and so forth. Rats who have a companion live a lot longer healthier life than rats who are kept as singeltons. I breed rats, professionally for about 10 years or so now.

I would highly recommend Martins cages. http://www.martinscages.com/products/cages/rat/

The rat highrise or rat tower would be a great cage for only a couple of rats. Just please make sure to get it powder coated as it lasts a lot longer and you don't have to worry about toxic fumes coming up from the bars. Powder coated cages are very easy to clean. Just take them outside and spay them with a hose. Any dried food or poop should rinse right off. If you can afford it, the The Ruud Cage (R-699), is my fav!

Another good cage company is Quality Cages. They are a lot sturdier than Martins cages but more spendy. They are based out of Portland. They also make custom cages. Plus, their running wheels are awesome!
http://www.qualitycage.com/rat.html

Make sure your rat still have a solid surface to rest his tootsies on. Rats love hammocks! Any other questions? Just FYI, ferret cages are not too safe for rats due to the bar spacing. You need to make sure their feet cannot get through the bars, or you can have an injured rattie in no time.