Update
So my first box of "dead" batteries arrived, $30 for 95 batteries (+ expensive shipping), I sorted them a bit, and I picked a handfull to tear down.
Results so far:
about 2/3 of the packs are recent-looking 6 cell Dell or IBM/Lenovo batteries, the rest is HP/Dell/Toshiba 8 cell packs, and a bunch of randoms.
A few of these packs are severely rusted on the inside/ outside. I'm going to assume they're toast. Most of the rusty packs are very old-looking anyway, no big loss there. I might open them to confirm (and take any non-rusty cells) but I don't expect much,
Another few (I think 4 or 5) are not 18650 cells, but prismatic (li-po?) cells. Also useless in my case. I might see if the ones that aren't giant balloons still take a charge, if so, maybe use them for something else.
While disassembling I started ripping the spotwelded tabs off. This might not be the way to go, as I got 1 cell punctured (a tiny pinhole in the bottom) and it started oozing blue fluid, so I put it seperately, I should probably put it outside.
Most of the spotwelds come off easily though, but some just tear, leaving little sharp shards of tab stuck to the cell. (I really should start using pliers instead of my hands)
Other than that, my nickel plated cell tab strip arrived.
And I missed a relatively good deal on those battery pack spacer clips, someone on Aliexpress was selling them for what amounts to $0.13 per cell in the finished pack (against $0.20 regular price on eBay.) but I'm still not convinced, so I let it slide..