I am not sure why you want to get rid of the hot water system. Baseboard radiator systems are widely regarded as the most comfortable of heating systems. I would suggest leaving it alone. If itís because you donít want oil, you can convert it to gas.
If I may also suggest:
Keep or upgrade your hot water system, then add a/c in your basement. It should take up less room than a full heat/air system. Also, there are new a/c units which only need a 2Ē ducts for intake and returns. You really should not go with one return for the whole floor, as the closing of doors in rooms, etc., will cause uneven heat, and a common return is usually much noisier.
It sounds like you want 2-zone, so you will want to add a second system in the attic. Same approach. These ducts will easily run between joists, or in corners of closets. The ideal location for the vents is in the ceilings, along the outside walls, over windows if possible. The returns can also be in the ceilings, but by inside walls. In this way, the cold air will fall down, while the hot air will be sucked up to be cooled in the a/c unit.
You could start out by just doing the second floor for a/c, if you want to spread out the cost. Then, add a/c to the first floor, and finally, upgrading the baseboard heat. If it is still functional, leave it alone until it goes out. If it is shot, then you obviously need to address that first. But, in any case, I would at least seriously think about keeping baseboard heat for your home. I whish I had it in mine.