Your situation was the same as mine when we had our home built in 1977, heat but no AC. Had a unit installed the following summer, (we moved in Oct, and house is 1800 sq ft). We had a 3 1/2 ton put in, which lasted until 2010, when we had a new furnace and AC system installed. Our furnace is also in the basement, and there is NO, problem moving the air, although we do change vent position for summer and winter, as cold air drops, and heat rises. At 1500 sq ft, you would most likely need a 3 ton unit, a 13, 14 seer would probably work, BUT, if the humidity is high, I would check a higher seer, 2 stage unit, as the first stage runs on low, keeping humidity down, and when cooling is called for, the second stage takes over, this is probably the only mistake I made living in Chicago, when it is humid in the summer. If you subscribe to something like Angie's list, they can direct you to a good installer, friends of mine got a great installer from Costco, and it did not cost $5000, more like half that amount.

Ok - do you have access to existing ducts? Do you have dampers on existing ducts? You don't really need a/c downstairs, right? Because then maybe a few minisplits is the answer. Dampers are cheap (as dirt) and with some seasonal readjustment, are probably your best bet. A few new ducts perhaps to even it all out. You could put a system in that is sized to just upstairs and close all the dampers to the downstairs in the summer. Or the dampers could be mostly closed. Normally this might be an issue but if you undersize the a/c, I don't see why there would be a problem.

I think installing a condenser in your attic would be a big mistake. It will perform poorly, noisy, and difficult to service. Do any of the other townhouses have an outdoor condenser? Where is it located? A condenser can be installed above ground attached to the side of the building with a bracket. This is very typical for townhouses that have no adjoining land. You would install it at a height that you can walk under it, but not too high so it would be difficult to service. I would think a 1 or 1.5 size unit for your house in the Bay area would be sufficient. You need a load calculation to determine the proper size. It is difficult to say how much it will cost given it is a new installation in a more difficult set up. Stay away from the big box stores. Get estimates from independent dealers. You can save a little money when the temperatures are mild and rebates are offered.

Is this a single story home? Are there multiple rooms to be cooled? Mini splits work for open spaces. Houses built in the 70s typically were built with multiple walls. You would need multiple mini split heads which makes it expensive. You already have a nice heating system. The air handler and duct work in the attic would be a good cooling solution. Keep looking for more HVAC contractors. The first two have proposed expensive solutions which are not the appropriate in my opinion.