How big an A/C unit to add & SEER ?

We are looking to add a central A/C unit to our forced hot air system. This is just for the upstairs zone since the house already has A/C on the downstairs zone (don't ask me why the previous owner didn't do the bedroom level if they wanted to save by only doing one unit). I thought this would be pretty simple since the A/C refrigerant and drain lines are already in place from the original construction. The house is in MA.

I'm sooo confused. I started out planning to get three quotes but when all three recommended different size units with different reasons for the size they recommended, I just don't know what to think. A} says 2-ton based on space (also says copper lines are too small for larger unit); B} space would requirements are borderline between 2 1/2 and 3-ton but the ducts are too small for 3-ton. C} says 3-ton would be best and ducts and copper lines are fine for 3-ton.

It just gets more confusing, with the first three being so different I decided to get a couple more quotes hoping maybe a couple would agree with each other and I could have some peace of mind and decide between those that agree on the system.

Additional info, the attic is finished and the current furnace comes halfway out through the kneeboard/wall which is 4-ft high and there is room to move around behind the wall stooping down, there is a false cabinet built around the furnace where it extends into the room past the wall. There is fresh air coming in through the vents in the eaves.

D} Says we should move the furnace and basically build a room around it to provide enough combustion air - says we should do this even if we don't add A/C, says the unit is unsafe as it sits. By the way, he quoted a 3-ton.

E} Quoted 2-ton, just received quote today and no call back yet on message left late afternoon (probably took off early, I would too on a Friday if I could). Also suggested replacing one panel on false cabinet with a vent grill or louvered panel.

When called with follow-up questions on quotes and asked about combustion air issue, A, B & C all said pretty much the same thing, plenty of air from the vents in the eaves and the unfinished space and a vent panel in the cabinet would be like an open window bringing cold outside air into the room in the winter.

So to sumarize, I'm pretty comfortable that the combustion air is a non-problem but I'm still left with two reccomending 2-ton, two for 3-ton and one 2 1/2

How do I really know the proper size unit for my house if the pro's can't agree? Don't know of any did Manual J, hadn't heard of it before tonight finding this site, but all did take measurements and noted windows, etc.

Second question, maybe much easier: All quoted 13 SEER units with option to upgrade to 15 SEER unit for around $<EDIT> more, in New England with the A/C only likely to be running in the late spring and summer months, are the energy savings likely to be worth the extra $ for the units?

Any help apreciated.

Thanks