Our 15yr old AC just died, so we're looking at replacement options, possible upgrades.

Some info on the current system:
Old AC is builder's grade, 4 ton, probably ~8 SEER.
Not sure on furnace specs but it seems to be working fine. Ducts are separated into upstairs/downstairs and have manual dampers.

We have an open floor plan, 2 stories, ~2000 sqft, high ceilings (10ft?), and the living room is open up to the 2nd floor. It's always warmer upstairs, often 5-10 degrees more. (Worse in the winter.) We're trying to see if there's a way to reasonably improve the temperature difference between the two levels (like putting in a 2-zone AC system). Another thing to consider is that we don't plan on staying in this house forever...if I had to put a number on it, I'd say ~5 more years. So we don't want to invest too much into the new AC and not see any return when we sell the house.

Regarding zoning to separate the two floors:
I've had some guys tell me it won't really make a difference, mostly because of the air flowing upstairs from the living room so you can't really separate the zones.
I've had some guys tell me that zoning will help, and can be done with any AC (adding about $3k).
And I've had some guys tell me that zoning will only really make a difference if we use a two-stage compressor.

Regarding replacing the whole system vs. just replacing the AC:
Some guys recommended replacing the whole system, furnace and all.
Others said we have some life left in the furnace (maybe enough to where we wouldn't need to replace it before selling), so we can replace the AC and adjust existing equipment to work with the new unit.

So our options are:
1. Install a new system with a two-stage compressor, with zoning. (Most expensive, ~$10k+)
2. Install a new single-stage system, with zoning. (Slightly less expensive)
3. Install a new system, single-stage, no zoning. (Middle cost, ~$6k)
4. Install the AC only, keep existing furnace, no zoning. (Cheapest option, ~$3.5k)

Thoughts on the above options?

Also, our thermostat is currently located downstairs. We were discussing moving the thermostat upstairs to try to make it more comfortable up there. Another option would be a wireless or remote thermostat that we could move around the house as needed. Thoughts?

I've already gleaned a lot of information reading posts on this site. Your input is much appreciated! :-) And I'm still in the process of getting quotes for different brands/models, and talking to installers. I'm just trying to narrow down the options at this point.