Your 610 is a dangerous bike...I have heard of all kinds of frame failures. I'll take it off your hands and properly dispose of it for you

Why do you prefer the 520 over the 610? Just wondering, I don't have experience with either, myself. Do they both fit?
You can change out the chainrings. Most likely 110 BCD, which can be found for pretty cheap. Full 10 speed groupsets can be had for fairly cheap these days, as everyone is creaming over 11 sp. If you are going down to a 700C wheel, you have more tire options and can surely squeeze a 35-38 wide tire on, if not larger.

On Chain Reaction, you can currently get an 11 sp 105 full groupset for $400, then all you really need are your wheels. You should come in well under your budget.