I would say try and sell that boiler on Craigslist or something...you could get enough money back on that boiler to pay for the forced air system....that was a good find! Awesome!!!
I've always been right partial to the RUUD and Rheem water heaters. We have a few local plumbers around that swear by them. The one in my house has to be over 30 years old, and with some new elements in it, is going strong! I still want to replace it though, primarily because it is not very efficient, and could use better insulation.
A 2-ton unit ought to be perfect for that house. My house is 1000 square feet, and it has to deal with the humid Virginia summers, and it does so just fine. I would say upsize a half-ton for a heat pump to what works well for a air conditioner, and just cut the fan back...the heating can sometimes bee more of a challenge. I can't remember how many square feet that house had, but it looks to be about the same as my house. Your gas furnace though is the perfect size for this house to keep it plenty cozy, especially if you install new windows and insulation. Is the ductwork already in place? If so, you are a good ways there, but make sure you check it for leaks. Ductwork leaks can cost a fortune in lost energy! If not, then you may end up having to pay some money to have some "tin monkeys"(HVAC ductwork guys) to come in and build some ductwork for you if you are not familiar with building it yourself (it's not hard really, just labor intensive)
Good luck with the house there, looks like it's really coming together awesome! Having central air on the house will be a good selling point no matter if you rent or sell!
BTW, I hope you saved those cool kitchen countertops with the chrome edges!