If you can get it working for $1,100 you'll have much more machine than you can buy new for $2,500.
Beyond that, the real question is what your personal trade-off between money and time is, and how used you are to machine reconditioning.
If you think the reconditioning itself is a fun hobby, there's no way you can go wrong :-)
I'm surprised that you need new ball screws. Are they worn or damaged or...? Is there any other obvious damage? Is any tooling included? Can you run the machine and listen for strange noises?
A very well used series 1 went for $3200 in my area. I went and looked at it but passed on it because of the condition. It sold eventually, I'm not sure what it sold for, but it was probably close to their asking price as I know several other people were coming to look at the machine. It sounds to me that your machine is in similar condition.
The machine of course could potentially generate tens of thousands of dollars in the right hands, so what's it's worth will vary wildly.