The old line set (copper tubing) can be reused assuming it is the correct size of the new system. If not then it needs to be replaced. It is not the preferred method, but it is done where replacing the line set is difficult. The new refrigerant will be R410. The old refrigerant (R22) must be flushed out of the old line set otherwise you have a contamination problem. This is another reason why it is good practice to replace the line set. FYI - we can talk about prices all day long on this forum. Be warned the lowest bid usually comes with the highest risk. Finally this has been beaten to death, but let's make sure you understand the concept. You can use a 3.5 ton air handler with a 3 ton condenser. The important thing is the air flow is adjusted to the correct speed. The 3.5 ton rating is the maximum air flow, not the minimum. In turns out in this case the 3.5 ton air handler has better efficiency numbers than the 3 ton air handler. If the bigger handler can physically fit in the space, then you can use it. You have to then decide if the extra efficiency is worth the extra upfront cost of the equipment.

The east side of the house is a large great room/kitchen/dining area with 12 foot ceilings, many windows and most of the walls are exterior walls. It currently has a 4 ton unit and 100k BTU furnace. The west side currently has a 2.5 ton unit and supplies the bedrooms and baths. This side of the house has varying ceiling heights under 10 ft. He said that from the load calc that I was undersized on the west side and slightly oversized on the east side. So I could possibly get a 3.5 ton on the east side but I wanted to go with a 2 stage and they don't come in .5 size. I am no expert in HVAC that is for sure and have tried to educate myself somewhat but I still feel a bit lost. Everyone tells you something different. I also find it hard to find good contractors. This one I am thinking about going with comes with decent recommendations from a friend and I like the AS brand.

Similar systems...I will assume you are being sized correctly...I would like to see a load calc performed to confirm this. 2 stage AC condensers with 80% eff 2 stage var speed furnaces...on the Carrier, I never like to see an Infinity furnace quoted without the Infinity controller...yes pricey... On the Trane, you should insist on a true 2 stage thermostat that controls furnace staging based on demand, not a timer or a software algorithm that will always go to high stage whether needed or not. I would ask this dealer about the evap coil they selected. I think the two options provided by Trane dealer are on the high side. Needs explanation. Ductwork is in attic? You want a thorough inspection. And I always recommend a pleated filter media cabinet regardless which system you decide on. Of course there is a pricing and warranty difference. Not sure if either brand has any applicable spring rebates.. And then there is the confidence factor you have with the dealers and their reputation in the location they serve. Are dealers authorized dealers? IMO

If there has been a coolant leak, after being turned off, it may not cool at all next time. I think that is quite possible. Again, I have lived with 100 degree plus outside, lost AC comepletely and not had it take that long to cool down once repaired -- even with high heat and humidity outside. The last time we were without AC, we got to 95 inside before we got a portable unit to help until we got things worked out (we had two warranty repairs and the second time the part failed -- then the manufacturer came back and said they weren't paying for the coil because the wrong part had been used and we got caught in a three way fingerpointing. We didn't know who to believe and finally found a new company to come in and redo the whole mess -- but it took 2 weeks to resolve and the cloudy, rainy days had given way to a heat wave). I get a hot house. I get it can take a few hours -- but 18 hours is way more than a few hours. The system may be freezing up and not working properly if it is running that long. The coolant may not be sufficient. The system may not be right for the home and it's construction (I was going to suggest that 8400 sq ft might need more that 2 zones to control properly, but it looks like a significant portion is your indoor pool and that has it's own system . Two might be enough, but something's not right. Glad you are questioning it and I hope you get it figured out. There is a school of thought that turning your climatr control off completely is not good for the system or the house itself. Your climate may be less extreme, but at least here, I've read things recetnly that suggest keeping your temp within 10 degrees at all times -- not just for comfort but also to maintain a controlled range of temperature, humidity and expansion/contration of all the structural elements. That's new to me, but it makes a certain amount of sense. Again, age and construction may have something to do with how much that maters -- just throwing it out there for you to consider.