While I have about 5 different megadrives in my collection I don't own any of it's lifesupport addons, even though I did get a copy of the 32X's Doom just because it was at a car boot sale.... one day I might buy the addon to play it lol.
From what I know about the SegaCD or MegaCD is that they are a technical disaster of really low quality CD drives which were originally designed for Audio CDs and not CDroms (or something to that effect) as such the chances of getting one in really perfect working order which hasn't been refurbished now is pretty low, also make sure you get the right Mega CD addon for the Megadrive you have, the Megadrive 1 version of the Mega CD is the huge platform which goes under the system and the Megadrive 2 version sits next to the console, they're not interchangable, so get the right one or you'll be going off to buy a second megadrive down the line.
Wait.... after taking a look around... and spending more time than I want to admit (refund please) you can use a Megadrive 1 unit with a Mega CD 2 (just needs an extender platform for the console to rest on) I think you might be able to use a Megadrive 2 with a Mega CD1 but it appears there is an extra cable needed to link them up instead of the base connection.
Also from what I read you want to get a Mega CD 2 because the first model of it has a mechanical opening disk drive rather than a top loader and yeah... it's old and more moving parts which all add to the chance of it failing to work.
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