Hi Retro; 

Spot welding looks like an excellent way to connect new cells, I remember one video where someone used a couple of car batteries and a momentary switch and got really nice results.  

However as these are already used cells, they will fail more quickly.  I've had to replace a cell three times recently from the 20 cell battery that I've been using about a year.  

The spot welding is harder to take apart, they come that way in factory made batteries and you have to pull or cut them off, which usually leaves some metal still attached.  For this reason it is better to use solder for older cells in my opinion.

As for safety, I am tempted to clamp my solder iron to a cell and leave it on maximum heat for 5 minutes to see what happens, probably nothing spectacular.  I only heat for about 10 seconds or so max because I don't want to damage it's capacity.  The metal of the cell itself acts as kind of a heat sink so it doesn't get as hot as a small component would, and soldering irons are built to melt solder. I've done over 100 solders on to bare cells without mishap or loss in capacity. 

The main danger is cutting yourself while taking apart a battery pack, the conductor strips are thin and very sharp when you cut them.  Other hazards are shorting stuff out with your metal cutter and seeing bright sparks. I once punctured one of the flat cells by accident and it started to get hot and smoke and smelled awful, but I can't see that happening with 18650s.