If I understand correctly you saw single color GTO's for around 6K mark. If that's the case then no, you cant link those GTO's.
If you buy a single color GTO, you will have to run a four color Job 4 times (one color at a time).
In GTO there are two different size models, GTO 46 and GTO 52. I would prefer 52 for the bigger size and resale value.
Also there are many different dampening systems installed on GTO's like, Alcolor, Conventional, Kompaq, DDS etc.
My advice would be to buy the Alcolor version as its Heidelberg original dampening system and very easy to control and master.
Some GTO's are installed with numbering and perforating units too which might be helpful in certain jobs.
I'll suggest you to get the machine inspected for any mechanical or electrical issues before buying. You can hire a former Heidelberg engineer to do this or at least someone who knows the stuff.
Don't forget to do a print test before buying if machine is in production.
In my opinion year of manufacture is less important, though try to find a press from 90's. A press of an older year but well maintained is better than a newer press if its not maintained properly.
And if I'm not wrong you can also get a Komori Sprint in this budget.
hope this helps.