Bigger dilema with the larger chillers than the smaller ones.
imok2 is still correct that you need to do a cost comparison for the two options. You should include estimated operating costs for both options.
Hopefully your contractor for the chillers will have some
efficiency figures for the...
For the oil level going low you will need to add an artificial load. tombmech is correct that you may not need much more load to keep the oil level from going low.
We had purg units on all centrifugals and I suggest you try to get funding for them.
What size are the chillers and how many?
imok2 has a great suggestion.
I am not an expert but have spent many years maintaining
chillers for air washers and process chilled water.
imok2 is correct in the assumption that "usually" the maintanence on chillers is at best adequate.
If the units are...
I must agree also that two units is preferred.
You don't give square footages of the upstairs and down but
you will be much happier with the management of temperature
and humidity by having two units.
What you were told about one unit being capable of handling the load of both stories is true...