Some more information might help. What kind of climate are you in? The utility of zoning might depend on your lifestyle if you are by yourself in the house and how many rooms there are. Is the basement conditioned?

Most of today's major brands offer quality systems and 10 year equipment warranties. Look at York, Trane and the other major brands. The expertise of your dealer/installer is just as important as the equipment you buy.

You might want to consider a 10 year parts and labor warranty since it will be a rental. And do encourage the tenant to change the filters regularly. Consider providing the filters for them.

Is the space currently comfortable? Is there ducting in all the rooms where you need conditioning? if so then replacement with a good entry level system will probably do fine.

If your climate is very cold, consider a 95% efficient furnace. That can save on gas usage. See if your local utility company has any special rate plans and see what equipment is required for them. In many states, rental remnants who pay the bill on an individual meter qualify for these plans, so check the specifics.

I am located at the northern border of humid subtropical zone.
There are two people who are going to live in the house. One will be out of the house all day, the other will often work at home. The first floor is essentially open plan of three contiguous rooms The second floor is two large bedrooms and two baths, and the third floor is a large room with a connected outdoor space. I don't know if the basement is conditioned but I tend to find conditioned basements too cold in many houses,,,I grew up without central air (in a cooler climate) and prefer open windows and fans except on the hottest of days. (This is somewhat of a difference of opinion in the household...what "hot" is)

I have only been in the house a couple of times, although during one walk through it was a hot day, the A/C was set for a fairly high temp and the house was not too hot, even up on the third floor. The house is mostly unoccupied, it has been used for several years as an occasional pied-a-terre by the owner--so typical current utility usage is very low and impossible to really extrapolate to daily usage.

I would go as efficient as possible but I also want it to be a reasonable investment.

If the place would be tenant occupied for a short stretch of time (6 months or less), I would defer making changes until making the residence owner occupied.

After that, shopping around for a good installer would be the first priority as installers are usually more comfortable with a particular brand (westinghouse, carrier, trane, ...).

I would also significantly improve the home insulation to minimize the need for cooling and heating.

The others have covered it well so far; you can find some additional tips here

-Bill