I used Cool Power --
www.coolpowerac.com -- to install my units and the duct work 13 years ago and they have run like a charm ever since. Our house is large as well, so we ended up installing two units, one for living areas and a slightly smaller one for bedrooms. I can't speak to your other questions, since I haven't dealt with this in some time and my information might be outdated. However, I still believe Lennox is considered the best. That's what Cool Power recommended and, as I said, not a problem in all this time. The only maintenance is a a tuneup and inspection every year or two at about $150, as well as periodic changing of air filters.
I believe LIPA has lots of AC information on its web site and may even be offering rebates.