If you are making your own buildings the dimensions will be down to you personal preference. I've made single storey buildings that are about 2" but they didn't have usable interiors. For buildings where the interior was accessible i've made them about 75mm so pretty much 3". I think 3" probably looks better and is pretty close to the heights of the official MAS scenary as well so if you did get any MAS buildings anything you've made wouldn't look too out of place.
I don't know about prices in dollars but usually a starter set and a couple of extra blisters or a starter set and one of the box sets are enough for a couple of 100 points worth of models which should be enough for a while if you're just starting out. But if you can resist the urge to buy more then i envy your willpower!
I think most of the basic faction starters are prety balanced except for combined army and aleph which have a lot more points than the other starters. I would probably avoid using sectorials to start with as well and their starters aren't always the most balanced.
The rules, army lists and army builders are all on-line and for free, yes!

So if you want to the only thing you have to buy are the lovely minatures!