From experience, it just seems logical to lower the car first so you know how it sits and then go for the aftermarket rims. If you do it the other way around and buy the rims first and then lower the car, you won't know how low the car will sit and you could end up having to roll your fenders (which costs $$$).I am very seriously considering getting aftermarket rims for my Genesis sedan in the very near future. My car is the stock height and would have a larger rim on it, possibly an 18" or 19" rim. Would it look funny not to lower the car first and get rims or should I be alright?
Another thing to consider is whether you'll be lowering your Genesis on springs or coilovers. With the coilovers, you can adjust the ride height, so you can either get the rims or coilovers first - it doesn't really matter. With springs, you're fixed to a specific ride height - the springs will tell you how much of a drop you'll get in the front and rear.
I would talk to the tire place where you're getting the aftermarket rims and see what the largest rim size they recommend is. This will save you from other problems like fender rolling and tire rubbing in the future.
Also, if you're lowering on springs, be sure to replace your stock struts as they were not meant for lowering springs - I learned that the hard way and blew my front struts completely... not fun at all.