Nah, you got it right.
Go to wheeldude.com and check out the selection and give them a call, they should be able to help you dial in a size. I've had Rotas before and they are good budget wheels, they sell a ton of them. The tire selection is kind of meh though if you do a package, or was when I was shopping.
You'll also need tuner lugs (the base on OEM lugs do not work for aftermarket wheels). The Muteki SR48 are cool looking opened ended extended lugs nuts (see my avatar) but most tire/wheel packages will include lugs.
Figure out your factory size, for me it was
Stock Wheels: Enkei (GSR)
Wheel Size: 18x8.5 +38 offset, Center Bore 67.1
Tire Size: 245/40R18 Yokohama Advan A13C (MY2008)
Comments: Cast, wheels are 22 lbs; wheel + tire 46.5 lbs
Then lay out what you can fit, with or without modifications, for example my set
Enkei NT03+M
18x9.5 +27
19.9lbs each
265/35-18 Continental DWS 25lb each
45lb total each
The weight matters. Unsprung weight is a killer. Buy heavy wheels and you'll feel it, it sucks. They should be at least your OEM weight, keeping in mind the increased width in tire and wheel. I lost 1.5 lb on each wheel but went up in size nicely.
I could have gone 18X 10 +25 and 275/35-18 but I would have had to have gotten my rear fenders rolled, (+$150ish) and had rubbing at full lock up front. It seemed a bit impractical, although the 10s have a little more of a dish'ier look. I could have done 275s on the 9.5 but went with the 265s instead, at one point the amount of contact patch will start to eat into your MPG and hydroplaning plus snow traction can start to be an issue. Reputedly.
Are you going to re-use your factory tires? Are you going to do a dedicated summer tire set? I live in MD so I went with a "high performance all season tire". Whatever route you go, do your research and pick a good tire. Tire rack is perfect for researching. In fact I bought my set from TireRack and got a good deal, they couldn’t touch anyone on price per wheel, also price for those DWS. Mounting/balancing is included as well. Also got the lugs (which I didn’t use) and hubcentric rings for free.
It seems like a simple upgrade, but it really takes a lot of research to see what's the best bang for the buck, then what you like aesthetically. Also if you find a wheel you like google that name and bend, or damage. I almost did the Enkei RFP1 because they are light and have a ton of sizes, but read some complaints about how they bend easy. The NT03+Ms didn't have much out there as far as being bent. Plus they are sexy as ****.