With Canon dSLR bodies, the sweet spot typically tends to be a prosumer APS-C body, one generation back. So, in this case, the XXD body that's one generation back from the current one (once a 7D successor arrives, this could include both the XXD and 7D lines). While the sensor and processor will be older, the range of usability features and the two-wheel controls with top LCD, and hardier build, make for a better set of features over a brand new dRebel XXXD model.
At the time I'm writing this, that would be a 60D body, used or refurbished. Typically, getting a factory-refurbished XXD one-generation-back body costs roughly the same as a brand new latest-model XXXD body. (Again, at the time of this writing, Canon USA's selling refurbed 60D bodies for $576.99, and a brand new 700D/T5i body from B&H costs $599.00).
Full frame bodies now have lower-end lines, and once there is a successor to the 6D, then the 6D might also be included, but full frame tends to have a "hidden" expense in that it can only use full-frame glass, and typically has a high-enough resolution that higher-end glass is a better fit, so as a "bargain" buy, it's unlikely to be as good as APS-C bodies.
As for the CHDK, that's more for Canon P&S cameras; for Canon dSLRs, look at Magic Lantern.