What size mill are you looking for, and what are you going to be cutting? Do you already have the motors for the conversion?
If the timeframe and quality are that critical for you, I would seriously think about returning your electronics and getting a used CNC machine or something small/cheap like a Tormach 440. It seems like you will be spending a good bit of time setting up your new mill and chasing down issues before you're ready to build. Further, if you're buying a mill and doing a conversion, generally you'll need ballscrews and conversion kits which will be another 1k or so.
Based on the prices you've mentioned, I'm assuming you're looking at smaller mills. If you're still set on doing a conversion, I would recommend looking at Little Machine Shop. They also import the Sieg mills, and I think they have a good relationship with the manufacturer. They also have CNC conversion kits for their mills as well.
Good luck!
Micah