As for buying the SD60MAC and converting them to DC:
I suspect it would be technically feasible, since an AC locomotive seems to differ from its DC sibling largely by having ADDITIONAL equipment. (I know a bit more about the GE design. The AC44 and the C44-9W have the same engine, producing AC current by turning the same (I think) main alternator and feeding it into the same, or at least similar, rectifiers-- after which the C44-9W sends the DC to the motors, and the AC44 sends it to the invertors.) So the conversion would involve (i) ripping out the invertors, (ii) replacing the traction motors, and-- this is the fiddly bit-- (iii) replacing the control system.
Is it worth doing? You'd end up with 4 locomotives with non-standard specs, and the big railroads would prefer not to complicate their maintnenace and record-keeping. But if you knew of a SHORT-LINE that wanted 4 SD60/SD70-ish units but didn't want to pay full price...
Can you get an SD70 without dynamic brakes? E.M.D. will sell you what you want, but these days even railroads with flat profiles tend to think of dynamics as an essential aid to train handling.
Are you, by any chance, a multi-millionaire who has secretly bought the Florida East Coast? Or were you questions asked out of pure curiosity? (Grin!)