The Macintosh 128k and Macintosh 512k were the first 2 Macintosh models ever made. I could go into a profound paragraph on how revolutionary they were, but you've all heard that before, so I'll just put some basic info here instead.
I'm going to treat both models as basically the same because the only real difference is the RAM amount.
The most common upgrade for an original mac is the 512k upgrade. Most software required 512k RAM, and Apple offered a logic board and badge swap that got you essentially a Mac 512k. Most 128k users had this upgrade done, officially or unofficially.
If you wanted to stop dealing with swapping floppy disks, you could get an absolute mad hack of a hard drive upgrade installed. This upgrade gave an expansion board for interfacing with an MFM hard drive, a drive, and a second power supply. It did work though!
The original Mac keyboard is pretty rare nowadays. It was missing all keys except for the main typing keys because you were expected to use the mouse for everything else. It used a phone jack cable to connect, but it is wired differently to a normal phone cable. Using a regular phone cable will fry the keyboard controller. The mouse used a different proprietary connector exclusive to the original Macs, and the Mac Plus.
A second external floppy drive was available to help get the constant disk-swapping down a bit. It plugged into a special floppy connector on the back of the mac.
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