Thanks to everyone for your input on my prior post:
http://ths.gardenweb.com/forums/load/hvac/msg0414214513570.html

Based on the feedback from the pros so far, I see that next steps are as listed below. Please let me know if I've misunderstood or missed anything:

1 - Ask dealer to provide, in writing, load calculation support for a 2.5 vs 3 ton system

2 - Ask dealer to provide, in writing, that existing ductwork (both supply and return) is sized correctly to handle the additional CFMs provided load calculation indicates going up in size to a 3 ton condenser.

3 - Ask dealer to provide, in writing, that existing lineset is sized properly to manufacturer spec and ask dealer to warranty any future issues with lineset (critical parts are connections at each end) $1000 to replace is not advisable unless existing lineset is not properly sized.

4 - Get additional quotes on equipment noted in proposal #1 from other Carrier dealers (along with load calcs). Current dealer pricing is very high. Remove Greenspeed model 25VNA036A003 from scope given we have a gas furnace for the first floor (this is overkill)

5 - Existing proposal for backup heat model FC-2901N09 is satisfactory (9KW that is field convertible to 3 phase) Ask dealer to install staged heat strips and specify in proposal.

6 - Continue to use current 4 ton central A/C until it dies. Not necessary to replace at this juncture.

7 - Discuss lock out temperatures with installer to ensure proper energy usage.

Some additional comments/concerns:

@tigerdunes - in response to your last post, we are running emergency heat strip intermittently for heat. I have been turning the fan on upstairs to draw heat from the first floor furnace to keep temperature comfortable on second floor (around 68F) Thankfully, there hasn't been a need to keep emergency heat on constantly.

@tigerdunes - thank you for suggesting additional equipment. In your opinion, is the incremental cost of the two-stage Infinity 25HNB636A003 (proposal #1) worth the investment vs. the lower priced systems you suggested? I'm trying to make the best cost/benefit decision since we do not have any plans to move from this home. If it were your home, what equipment would you choose?

@mike_home - you noted early on that the proposal pricing for #1 seemed high @ approx $7K. Is there a price range that is more reasonsable for a 3 ton system provided everything else remained constant - same backup heat, infinity controller, using existing lineset? What about a 2.5 ton system? (i.e. $5-6K for 2.5 system is fair, an additional $500-$800 for a 3 ton system, etc)

@mike_home - you recommended getting a 4 inch media filter. Dealer proposed upgrade to a Aprilaire 2210/2410 Air Cleaner for $300. Is this worth the additional investment or do you have another suggestion?

@ryanhughes/tigerdunes/all - you indicated the existing VP 8320s could be reprogrammed and used. Since they are full touch screen models that appear superior to the SYSTXCCUID01-V Infinity controller proposed which isn't a touchscreen, I was curious to the advantages of replacing vs. using the existing ? What are the drawbacks? I'm very pragmatic about changing my filters every 3 months so I don't need a controller telling me to do so if that's the only advantage.

@ALL - Should I only pursue Carrier dealers at this juncture for quotes on equipment? Since I have multi-speed Carrier 59TP5 Performance Series 95 gas furnace, I am assuming you all are recommending that I do not pursue Trane/Lennox or another manufacturer as it would create a mismatched system? This was the one point I didn't hear a lot of feedback on other than the Trane having true electronic demand defrost.

Again, MANY THANKS to all of you for the invaluable advice.