Luckily, I do happen to know that stuff, so I used the existing board with brand new 18650 cells. Unfortunately, the board seemed to brick itself when it lost power, so the vacuum kept complaining the battery wasn't kosher.

>This is only good advice if you're good at soldering

I meant soldering onto the pre-welded tabs that come with the new cell (unless you have a spot welder). You don't need much soldering experience for that.

>and know details about cells like which ones have in-built protection.

It's highly unlikely that the individual cells would be protected ones. Manufacturers are not stupid to pay N times the cost of a management circuit.

I don't think you'll ever find a battery pack using cells with integrated low-voltage protection, if that's what you're referring to. All that stuff is managed by the BMS. What you should be on the look-out for is the cell's operating range, continuous and max power. Personally I use buy VT6's in bulk and never think about any of that.