Replace evaporator coils when replacing R22 condenser with a 410a unit

First off, I vote for a new unit if you can swing it, but $100 is a lot cheaper and you are not likely to offset the cost of a new unit with the savings from a newer, more efficient unit for many years.

The condenser fan motor isn't that hard to change, you just need basic hand tools and a gear puller. It would also be a good time to replace the fan start capacitor as well.

Using a single HVAC unit with zone dampers is pretty well-established and can save the trouble of having multiple separate units. Some things to discuss with your salesman (and if s/he doesn't have detailed answers, ask for someone with detailed knowledge):

• If your cooling needs will often only require one zone or the other (i.e., daytime downstairs, nighttime upstairs), will the system cool that single zone too rapidly to allow proper dehumidification?
• will it have the capacity to cool both zones effectively when needed? (balancing this with the first point may require careful sizing of the system)
• if the system fails, you have no cooling. With two separate systems, you're likely to have some cooling in the house with one system running.
• this might give you the option for a third zone, if your current ductwork allows (i.e., split one of the zones into two). This could be a cost savings if you are currently cooling space that's not used full-time.

We've been in a house with three zones on one system for 13 years and it's served us quite well.

I can't comment on that specific unit or the Nest.