Made a major purchase that I wanted ever since I first heard about the 550c from the Cult of Mac book years ago -
I picked up a Powerbook 550c with the box and manuals for a grayscale 520 for $350 shipped from Japan.
It had some minor cosmetic damage to the screen from a leaking PRAM battery ("vinegar syndrome"), unfortunately, but otherwise is in pretty great shape! Already upgraded it to 40MB of RAM and the original 750 MB HD works great.
As mentioned it was an official upgrade, replacing the ROMs and the IWM for a SWIM, usually replacing one of the floppy drives with an FDHD unit (complete with a sticker to go on the case) and leaving the other as an 800k.
They rearranged the acronym with the FDHD system: originally an Integrated Woz Machine, it became the Super Woz Integrated Machine. I guess it flowed better than SIWM would have.
I used to have the 72-pin to 30-pin expanders, but I gave them away (don't remember who.)
However, I have a bunch of the 30-pin to 30-pin expanders so you can use lower capacity SIMMs. I also have a set of SIMM doublers for the IIfx that lets you use 2 SIMMs per slot.
Don't use cleaning disks on 400k or other single-sided drives or you'll damage/dislodge the pressure pad and render the drive unusable. The pressure pad is a little felt pad that, on the 400k Sony drives, is on the arm on the top of the drive directly above the head. This pad presses down on the disk surface to ensure full contact with the head. If this pad is missing, it will not generate enough pressure to properly read the disk and may scrape up the top of the disk causing a lot of debris to be generated. Cleaning these drives requires disassembly and manual cleaning of the head with a lint-free swab and special cleaning fluid, but you can get away with carefully using a cotton swab and 90% iso alcohol. 70% iso also works in a pinch. You'll also want to check that the pressure pad is still properly attached. If it isn't it may be in the drive somewhere.
Even though your disk media may not look bad, it may be heavily shedding particles and/or binder material all over the heads of your drives. I probably wouldn't try using it again.