You can set up auto ISO, along with an upper ISO limit. A limit of 1600 is suggested. Beyond that you are going to get noise that you won't be able to easily fix.
To set up auto ISO, just use the usual ISO button and scroll up and down until you see "AUTO". Then click to the right and you can set min and max (my camera is now set to 100, 1600. 100 is as low as you can go by the way.
You can set up the AEL button in one of two ways (unless you decide you want to use that button for something else). Either "hold" or "toggle". I decided that I want toggle rather than hold (since I don't want to try to hold the button down for a series of shots). Go to "Custom Key settings", which you find in page 6 of the "gear" section of the menus (settings). Note that the choice shows a mysterious dot in a box for two of the choices. This is for spot metering, which is probably not what you want.
Tom's Digital Photography Info / tom@mmto.org