I have a 128K because I collect these things. It's kind of bordering on useless as a computer-in fact it's frustrating since at best you're juggling disks a lot more than with other computers, and at worst there's a decent bit of software that just won't run because of lack of memory.
The original 512k is more useable, but you're still a bit limited by 400K drives and such.
I consider a second drive a necessity with both a 128K and 512K. The disks are just too small and the the 128K in particular has too little memory to not have one.
The Plus is the king of the bunch, and there were SO many of them made that it's also the easiest to find and IME the most affordable(although I've never actually bought one-all of mine were given to me). Sometimes you can even find really early systems that were upgraded to pluses-most of the parts get replaced when doing that but it was an Apple sanctioned upgrade. I have one that has I think a March 84 SN-it's definitely early.
The Plus in its as-shipped state has 1mb RAM, twice as much as any of the others, and can go to 4mb. It also has SCSI, which makes life easy for external storage(although getting one to boot off SCSI can be...fun).
I'm rather fond of my 512Ke, which(as best as I remember) is Plus ROM but sort of "stripped down" to 512mb soldered RAM. Mine was my first M0001 Mac, and I've used it a lot. Mine came to me with a 512mb RAM upgrade "piggybacked" on the soldered RAM to give 1mb total. All I think have 800k disk drives, which frees you up a lot, even though I still have a drive in my set-up. There were upgrades that added as SCSI port to the 512Ke.
BTW, I run my 512Ke and one of my pluses off an HD20. This is the original floppy port hard drive. I actually was given two of them from the same source, both of which were paired with plusses, although they trickled into me over a about a 2 year period and the one got "adopted" by the 512Ke before I had the Plus it was originally paired with. I have no idea what you'd have to pay for one of these drives, as I don't see them for sale terribly often. You can boot a 512K, Plus, and even SE off of one(maybe a Mac II also? not sure). With that said, the 512Ke/Plus ROM will boot directly from them if you just plug them into the floppy port and power the drive on before the computer. On a 512K, you need a "boot helper" disk(I have one that I need to image since I'm not sure how available the image of it is) that the computer boots off of and then you can further boot from the drive. I've not tried, but I don't think the 128K is compatible at all with it.