Include the basement in the load calculation now, and I'll bet you'll find it adds little to the sizing requirements (cooling wise at least). My basement has a few "taps" into the main trunk line with no real issues. I'm the first to admit this is not the best way to go. The bottom line is, the basement will almost always NEVER be quite as comfortable in the winter as you want it unless it is zoned off the main system or has its own system. I know that is the case for mine. You know it's a cool basement when you walk down the stairs. If you just add taps into the existing ductwork, make sure it doesn't starve the main home of its supply air. This may mean upsizing a bit, but it's likely already "upsized" a bit as most new construction installations are. What size system is going in as of now?

moonsoon first and foremost, as said in my previous post, you need a load calculation in writing broken out by each zone. I believe 10.5 tons is grossly oversized. EACs refer to electronic air cleaners which I believe the GAPA is Carrier's model. You need some type of air cleaner for both systems. EACs are not a cure for asthmatics but may help. Don't know about the GAPA model but most require regular cleaning/maintenance and do produce a small amount of ozone. I would not have one personally. I have a 5" pleated media filter that I change once a year. I lean toward the KISS system(keep it simple stupid!). No offense intended. The "return" air recirculates your inside air. It has to be sized to be in balance with your "supply" air. For example, if you have a 4 ton blower, your ductwork system should be sized accordingly-1600 CFMs both supply air and return air. One large return such as listed in your original post with such a large space will give poor airflow/exchange and most likely will be noisy. That was the reason for my suggestion for another return. And definitely don't place it in a living area-they should be centrally located in your home normally in an inside hallway. again I know that electric rate is not correct but I will not beat a dead horse. I do think a homeowner should have an apprx idea of their utility rates. what are the tier electric rates? I don't have a strong opinion on UV lights. Most homes don't have them and I would think they would be better indicated for areas with high relative humidity or to focus on specific problems with your evap coil. You do want Carrier's tin plated coil not the copper models. as far as ductwork, metal main trunk lines that are insulated with flex for supply and returns would be my choice. check with county building office and see what code is for new construction. IMO

your hvac systems are oversized. all bids come out to about 400 sq ft per ton. hugely oversized for a well sealed house. where are you putting the foam insulation? better than filling stud cavities in walls, using 1" foam sheathing to exterior, conventional insulation & airtight drywall approach to interior is a better more affordable high performance wall. save the foam for the roofline & make sure that inches = R-value for your area (R-38). no "performs like" R-38..inches = R-value. you need a load calc, seriously. there is a guy fairly close to you who does load calcs, blower door & duct testing & has ties for foam insulation company. drop me an email & I'll share his contact info with you. don't pay for oversized equipment that will run in low speed 95% of the time. take the time now to get units sized based on accurate inputs into load calc. ductwork is in foam insulated unvented semi conditioned attic correct? paying upgrade from R-6 to R-8 isn't much...but may be beyond what install crew has worked with. not a huge issue, but you'd be surprised how they whine & moan over stupid stuff... just fyi other posters...code for La. is R-6...not R-8. crazy, but there it is. and OP, take a look at Slemco's Design One new construction guidelines. best of luck

Now that you have discovered you have natural gas, are you going to investigate who much it will cost to bring a pipe into your house? I don't see in the load calculation the R values for the walls and attic? Do you know what values were used for spray foam and batt insulation? Do you know how the spray insulation will be installed in the attic? Will it be open or closed cell? The firs quote is for a 4 ton single stage heat pump (XR15) while the second is for a 5 ton variable speed heat pump (XV20i) which is Trane's top of the line product. It is not a fair apples to apples comparison, but the price gap is big. You need to ask about the size for the second quote.