The MD1 can relatively easily be installed with a region and 50/60Hz switch. I'm unsure if a modded PAL system will output a proper NTSC signal, but with a little googling you should be able to easily find the information you need.
As for region locking, originally, no MD games were region locked. Starting in 1993, region locking was introduced little by little, and before long it became the norm. There are a few later games that are cross-compatible but generally it's safe to make the assumption that if a game was made in 1993 or later, it is most likely region locked. Any older game will work on any system, but note the following:
-Unless modded, PAL systems will always play games in 50Hz. This may also affect audio (IE slower music)
-In the older, region-free games, language is generally decided by the console; IE US and PAL systems will run games in English, and JP systems will run games in Japanese (much like PAL systems will run any games in 50Hz, and NTSC systems will run them in 60Hz). Of course, this does not apply to games that were only released in English or Japanese.
-Japanese Megadrive cartridges are different from western ones, and won't fit in a US or PAL systems without modification to the game cartridge or the cartrige slot on the console. Japanese systems have a wider slot, so US and PAL games fit fine in Japanese systems.
