The T2i-T3i-T4i share the same sensor. Image quality is similar, except the T4i may be slightly better due to better noise reduction in the processor. Canon's been lazy in this regard, rehashing the same sensor over 3 generations.
I highly recommend keeping the 55-250mm IS. I think it's Canon's best bang-for-the-buck lens along with the 50mm 1.8. In fact I would take this lens over the next step up (Canon 70-300mm IS, Tamron 70-300mm VC) due to similar performance at a smaller size/weight/cost.
Canon's kit lens is also very good, one of the best in image quality among all the manufacturers' 18-55 kit lenses.
This and the $290 Sigma 30mm that's also on DOD should be a great setup for anyone under $1000.