1 - 80% AFUE (non condensing)
2 - 90% AFUE or higher (condensing)
1 - Fixed speed blower
2 - Variable speed constant torque (AKA X-13) blower
3 - Variable speed constant airflow (AKA variable speed) blower
1 - Single stage gas valve
2- Two stage gas valve
3 - Fully modulating gas valve
Above are pretty much your options when it comes to a gas furnaces. Pricing is lowest at 1, moves up in price when you go to #2 and #3. AFUE% over 90 can run from 90% up to 97% to 98% which also increases furnace price. I can't think of any line of residential furnace that will not run either on NG or LP (as mentioned, generally comes with a LP kit which adjusts the pressure and a different piston).
2,000 square feet tells you something, but the because this is new contruction, are you planning on having a load done on the house with the various insulation factors (which can make a huge difference in load capacities needed)?
Anyone can ballpark it for 3.5 ton but depending on how well it's insulated it could bring that size down easily.
How many floor levels?
Are you planning on one system, two systems or zoning?
One thing to consider is to be proactive in the building of the house to ensure you have more than enough space for your HVAC equipment along with ductwork, particularly 10-20 years after the house is built and you need to access your equipment.
If going with ductwork, the more hard metal pipe that is insulated by being wrapped (vs being lined), the better IMO, but that also will raise the price.
Using 3.5 ton for an example (purely a guess given the size on how guys "ball park" it), if on LP, a 2 stage will offer you more comfort on the heating side because at 3.5 ton, most furnace sizes will be in the 80-100k BTU range. A two stage gas valve (not a timed gas valve that automatically goes into 2nd stage if heating isn't met within a certain time frame irregardless of inside temp) will give you a low fire and high fire BTU rate, generally without the furnace having to be on second stage (high fire) rate.
One other thing to think about if going with a AC unit is to dual fuel it, using a heat pump outdoor unit with coil, and use a furnace for "back up" heat. This way, when you do need heat when it's less than 65F outside air, you can run your heat pump for heat down to a certain temperature (say 20F or 30F, and then your gas furnace comes on). This way, you get the best of both worlds, using electric for heat until it gets "very cold" and then you use your furnace to make it more comfortable on the heating side. If on LP, your burning a lot of fuel anytime that gas furnace comes on for heat, so why have it (gas furnace) running the majority of the time when it's 30F-60F outside and the heat pump can maintain the heat?
Also note, if you wait until January 1st 2023, 15 SEER will be the minimum SEER rating in the US, and unlike heat pumps, it will be illegal to install exisiting AC inventory after that date. 14 SEER Heat pumps will be allowed to use existing inventory after the January 1st date, and we're still wondering exactly how the government/local government is going to enforce that LOL
