Decide on your tooth count, and buy the chain long. You may need an extra link in your chain for that 42 tooth sprocket - especially if you convert to a 190/55 tire.
Just watch out for the widths. 520 vs. 530. Make sure both sprockets and chain match.
I'd rather see you try a 15 tooth front - assuming you have the stock 16 now. It's the equivalent of going up almost 3 teeth on the back. It'll make a noticable difference, but not so much that you won't like it. Quicker acceleration, and shifting points are at a slower speed. You can use 2, 3, and 4th gear more efficiently in traffic and city streets/traffic lights.
I run a 15/42 here in Sacramento and the outlying foothills and canyon highways.
If your chain is stretched all the way to the back, replace it. Sprockets should be replaced whenever a chain is.
If you think $200 is expensive for a chain and sprocket kit, wait until you toast both engine case halves trying to squeeze some extra life outta that chain.
Do not buy vortex, or any other aluminum rear sprocket for that matter. I had a steel superlite sprocket and it was nice and light.
Not sure why you're condemning all aluminum rear sprockets. AFAM makes a damn good rear sprocket, I got about 20,000 out of my first one, and replaced it with another identical AFAM. It weighs roughly 1/2 of the same size steel sprocket.
And since we're talking about quality, I'm having trouble with my new DID ERV3X chain spitting up little strips of the x-type o-rings. Not the whole o-ring, just the top part of them. I lube and clean the chain no less than every 400 miles.