Trane doesn't have a low end model. All of the Trane systems have the same compressor and same brand parts as all of the high end models so the only difference is the efficiency, fewer options and the factory warranty, making them less expensive. The XB systems are the least expensive and only have a 5 year compressor warranty and parts and have no bells and whistles.
Placing an a/c upstairs is a good idea, but keep in mind that cool air drops and heat rises, so as long as the space between the upstairs and downstairs is open, this will happen.

I have a similar problem in a so called fully airconditioned Cape Cod - I have posted before on my problems. My problem is exascerbated by the fact that there is no return air duct upstairs, only downstairs. Hot air rises and so there is no way the upstairs is EVER going to cool. It is annoying that so called professionals do such an unprofessional installation job - total garbage. When I bought the "airconditioned" house A/C was a major factor and I had no idea that the upstairs would be a virtual furnace. The inspection report was also silent and thus negligent on this very important "living" issue. You dont say whether you have an air return duct upstairs or not.
My solution is probably going to be to add a mini-split unit for the upstairs three rooms. No ducting will be needed that way and the current A/C unit will do fine for jsut the downstairs. In fact it could run more efficiently if I shut off the air feed to the upstairs rooms in the summer. This will also defacto give the house two zones and at night the main A/C will be off as no-one will be downsatirs as the main A/C will no longer be needed for the upstairs if you get my drift.

Yes there are return vents, 2 in the biggest room and 1 in another room. 1 room doesn't have a return vent because it was an add on, and when the previous owners had the A/C installed they didn't pay to have that room hooked up, so we just ran an air vent to it (but no return vent). Even with return vents it is still bloody hot. I think a lot of it has to do with the small size of a Cape Cod attic, compared to a regular house attic. And the small size attic has me wondering where I would put the air handler if I installed a/c up here.

If I were to go with a mini split system like you are thinking about, how would that cool all three separate rooms? I don't know much about them, but it looks like you have 1 indoor air handler and then the outdoor unit. Wouldn't that be a problem to cool all three of your rooms? I am assuming you don't install one in each room, or do you?

After reading more I now realize you have 1 outdoor unit and up to four indoor units. This is something else for me to consider now. It would be easier than trying to figure out where to put the air handler in the cramped attic for a ducted system. Thanks for your help.

We are going to go the minisplit route as I still cling to the belief that a central airconditioned house is a big resale and value factor. I would never buy a house with a "window unit" or whatever. Of course the minisplit also gets us to where we want to be - sweat free in summer without the loud noise of window or through-wall units.

In fact we may go minisplit on the ground floor as well - that way we can pull out the ducts on the basement ceiling and have a worthwhile living space in the basement which is currently impaired.