I ran VMWare Fusion on a first gen Macbook Air 11, and now am running Parallels on a first gen Macbook Air retina 13", for exactly the purpose you had in mind. Both worked well. Compatibility is high.
For the Macbook Air 11 I opted for Windows XP, on the 13" I'm now running Windows 10. Your 'old Mac' still should be Intel based though probably.
In terms of speed, Win 10 now is slightly less performant than Win XP was on my old device (although Parallels apparently is more optimized).
If you get a Windows key from a shady ebay seller (usually sell decomissioned units versions, or mass activation keys), the main cost point will be either VMWare fusion, or Parallels itself.
I can just say - that this route works well. And worked well for me in the past.