There are several people on this site that have retrofitted mills and Tree 325 specifically. I bught a 325 cheap and decided my best approach was to remove all existing electronics and replace them. With my machine 70% of the work involved cleaning and cleaning them some more cleaning. Mechanically the machine was in good shape and looked to have been well maintained. new belts, no binding on the axes etc.
The basics of the retrofit came down to buying a computer, power supply, servo drives, spindle drive and controller software. I sold the existing electronics for about the same money as the replacement. You have to realize that patience is required. The spindle drive I got costs $1400 brand new but I was able to get mine for $325 on eBay. Although it was an "interesting" deal because I think the seller realized he sold it too cheap, but too late.
I can be more specific in listing the components I used but you must first decide how much you want to spend and have cash ready to go - realize that you will need more than you thought. Then decide if you will fix existing components or replace. Check all motors for operation because if they need to be replaced they will be one of the biggest expenses.
Because of my limited budget I stayed away from "pre-built" retrofits like MachMotion or Centroid kits because I could buy the components and go through the pain of integrating for much less. Ajax is a good source for these items but doing lots of research on the web will get you all kinds of info.