Upgrading the size of your wheels could be considered as upgrade, but it depends who you are talking to
In terms of handling/ridability the wheels have very little effect other than their given weight, that's all dependent on the tires.
The stock es300 comes with 205/60/15's so that should make the wheels close to 15x7's, I think, which SHOULD fit onto your car no problem.
What I would do first is look at your local craigslist, e-bay, etc to find someone selling these wheels. Fit one on the rear wheel and if the tire clears the strut and does not bump, you're in business!
And yes it makes sense! Rims can define the look of the car so any upgrade is good