You can probably find a converter and still use some of that old glass! The multiplier for Canons is 1.6 (Nikon, Pentax and Sony are 1.5). Still your calculations are about right. Or say screw it and get a full frame
Someone else will have to answer the other questions, my knowledge is more limited. I only use the 18-55 IS that came with my 450D for my "standard" lens.