Canon's EOS Rebel T6i, SL2, and T6 are all upgrades to your 40D, and cost less that $600 NEW!
In addition, the kit lens they come with is better than the one you got with your 40D.
I do not recommend used dSLRs. They contain many mechanical and electromechanical components that are subject to wear, and there's no way to know how much use and abuse a dSLR has been subject to. But if you go with used, buy from somewhere with a warranty. KEH has a special now where, instead of their 180 day warranty, you get a full year. They also have an EOS Rebel T6s in EX+ condition for $599.
- The lens is the thing.
- 'Full Frame' is the new 'Medium Format'.
- "One good test is worth a thousand expert opinions." - Tex Johnston, Boeing 707 test pilot.