If you go with Windows 10, make sure you don't get the home edition. Change the update settings to block feature updates for 365 days. Good for all computers in the office really.
If you're comfortable installing windows/replacing minor components here and there, keep on using those old machines. As long as you're not losing any data if they go down, I wouldn't worry about it. Like if you know how install an SSD, you could always do that with your main computer for the upgrade. If it doesn't take, switch back to the old/untouched drives.
SandyBridge machines are still very capable today (released 2011). i5-2500/i3-2100. Processors are getting cheaper and more capable as well thanks to AMD getting back in the game.
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