Welcome to the forum....
They are a big responsibility and yes, start looking now for a bigger tank. As they grow their home will need to grow. Keep in mind, 10gallons per inch of turtle as a guide to their tank needs. Eventually you will be looking at a very large tank with having two of them so when upgrading now, do all the research you can ahead of time and go as large as you can in purchasing a new tank so you will save money in the long run.
If you have a good filter (rated 2-3x the tank size) & you do regular cleanings/water changes, you shouldn't have to worry about the smell. Many people don't use anything extra to clean the tank except clean water. You can use vinegar if needed, and rinse well afterwards.
Not all RES take to change well, so needlessly rearranging their tank after you establish it properly should not be done. But each turtle's personality is different, so some will not mind a little tweaking here and there as needed. Just be sure to read through the main site and forum for all your turtles' needs.
What's the rest of your setup like, UVA(heat)bulb, UVB bulb, heater, basking area, basking temperature, water temperature etc?
My turtle was an impulse buy as well, but after reading up on his needs and providing him with the best home possible, I wouldn't trade him for anything. It is a little overwhelming at first learning what they need, so be sure to ask any questions you may have.