Be careful if buying a Japanese Megadrive console as there is a simular looking console that is an Asian model, Asian megadrive wil have (mod no. 1601-11, found at base of console, also stating its pal (pal-1)),
An Asian megadrive will play most japanese titles (not all) but will play them at pal speeds which is nearly 20% slower than the Japanese NTSC version, as well as all asian, i believe all u.k. / european titles and australian titles as well (have yet to find 1 from those regions that wont work anyway) will run on an asian machine.
If want a japanese console :-
then look for mod no HAA-2510 on the base.
There are some 2510's out there that have had a pal mod done to them though, (if the console has an aerial lead sticking out back its a pal mod) a pal modded jap console wont run any regionally locked games (japanese wont run for sure, have found every european title thats locked to not run also, and there fore believe the same will apply to other region as well?).
a japanese mk1 machine will run fine with a u.k. power supply (no need for step down converter, step down converter is only needed if using the original jap psu).
if have a scart socket, its worth buying an rgb cable in order to get the best quality visuals, even better get one with a stereo jack lead, to plug into the headphone socket (on a mk1 md) and enjoy having some bass with your punches on bare knuckle.