HVAC - Dual Zone question (floor, furnace, AC, heating)

[WhiteSox]

[SIZE=2][SIZE=2]Currently in the process of closing on a short sale. Long story short - buying a single family home that has been converted into 2 units - Looking to convert it back.

House currently has split electrical systems/HVAC/Gas. I have family that can handle the gas and electric issues - my biggest unknown is on the HVAC front.

First - I like the idea of 2 systems although for the overall size of the home, I think its overkill (Chicago does get cold in the winter and I do like the potential option of keeping upstairs/bedrooms warmer while not heating the whole hose at night).

1st floor = 1000 sf (2 bed -1 bath - kitchen/living room)
2nd floor = 900 sf (1 large bed, 1 bath, small kitchen/living room)

The home has duct for the 1st floor running through the basement and through the attic on the second floor (although the 2nd floor is fully finished - all duct appears to be in the ceilings)

Currently there is no central AC installed.

My questions are as follows:
- keep the zones separate or combine them? (is this even possible?)
- i know the possibility of combining them all deepened on how larger of a furnace i have, but lets pretend its larger enough - is combining them the most cost effective solution for me?
- If i keep them separate - I'm assuming i will need 2 ac units when i install central AC?
- is it possible to have 1 large ac unit for 2 zones?

Again - these questions are coming from someone who had NO clue about anything HVAC related (i have installed a programmable thermostat before!) so all apologies of they are way off base.

Also, I do know that all things HVAC depend on size of units/ducts/runs and things of that nature, assuming all things line up correctly - I'm really just looking for advice/suggestions and different points of view (hopefully from some folks who know way more than I do on this topic!)

Thank you in advance your you time on this.
[/SIZE][/SIZE]

[K'ledgeBldr]

Depending on the age of the house it was probably two separate units to begin with.
Don't fix what ain't broke!
They're two separate systems- 1 up, 1 down. Need A/C in the future- No problem, add "A" coils and condenser units, 220v lines for the condensers, etc., etc.