First I have to say looking at the prosine, they don't have a built in charger. I thought they did, but they don't. So you would either need to get an external charger, or solar panels, or something.
I would definitely make the system portable since most people wouldn't want to deal with it. Fortunately most furnaces actually have a 120 plug right on them and then plug in to a dedicated outlet right next to them. This is good because it is easy to just plug that in to your inverter without messing with anything. So you don't really need to do anything with that part.
Getting power around the house is a different story. You could let your electrician know you will be putting in a backup power source and ask them to put certain loads on a sub panel. Usually a sub panel can be run from the main panel or from a different source, like inverter, or most commonly a genset. This would be the only thing really left in the house, but the cost of adding a sub panel right away would be about ½ that of doing it after the fact. And you could let them know which outlets you want on the sub panel.
Yes you can start with a small battery bank and then if you need a larger one, replace it later. The catch here is you can't or shouldn't add batteries to an existing battery bank. Old and new batteries don't mix well. Battery banks are going to be the biggest pain in your system, and usually last between 5 to 15 years depending on the battery type and how you treat them.
Think of the inverter like a large UPS. You would take a wire from your main panel, run it through the inverter to the sub panel. If the inverter sees power from the grid it just lets it pass through to the sub panel to power the loads. Then if it sees the grid fail, it automatically starts and powers the loads in the sub panel, and once the grid is back up, it switches back to the grid. It also will disconnect from the grid if it sees low voltage or over voltage to protect you loads on the sub panel. I have mine wired like a UPS. Meaning there are physically plugs that power the sub panel. They can be plugged in to the main panel, the inverter outlets or the generator outlets. And the inverter can be plugged in to the main panel or into a generator outlet in my shop. Typically I leave my inverter plugged in to my main panel outlet and my sub panel plugged in to my inverter. The only time I usually switch it is plugging the inverter in to my generator outlet if the power goes out.
Take a look on Ebay. I know you could get a SW2512 for less then 2k new and about ½ that used. Just do a search on Ebay for inverter. The catch is you will see a thousand smaller 100 to 300 watt units. If you add "Trace" or "xantrex" or "prosine" to the search it will narrow it down a lot.
Going back to the type of inverter I think I would have to recommend a PS or SW line now because of the battery charging part. They would automatically charge the bank from the grid and a good rate and be fully automatic. A good 3 stage external charger will cost $300 to $500, so remember that when looking at prices. The disadvantage is it is hardwired and not as portable, but you can hard wire normal plugs to it like I did.
Now since most of you loads will be in the basement, you could avoid the sub panel and plug things directly in to the inverter via extension cords. That will also save you some $, but you loose the built in look, that is if you care. Again think of it like a large UPS.
Check out some of the links on my stuff page.
http://www.uwgb.edu/nevermab/stuff.htm
I like Mr. Solar and or Backwoods. I bought most of my stuff from them. On Backwoods there is a "newsletter" part where people sell their old stuff when they are upgrading, you can get really good deal there.