Well, I'd say you're doing the right thing by getting estimates. Then, based on the cost, you'll want to think about if you'll end up making your home so much different from the rest of the neighborhood that you'll never get your $$$ back (if you decide to sell and move.) Also, you'll want to at least investigate if it would be better to move to a bigger house than to do what you're describing. Ask yourself, "Who is likely to buy my home?" Based on the various ways you answer the question, that will tell you if what you're doing is 'a good idea'. I'm not really trying to disuade you from your project - I'm simply offering some 'help' based on your question. As you can tell, there are certainly a LOT of ?s and you haven't cut a hole in the house yet.
Most likely, you'll need to upgrade certain appliances. You may need a larger hot water heater, heating system, and/or AC system. The only way to know if this is the case is to have your NEW home resized, and compare the #s to what the different units will service. Sometimes, it's not worth doing a major renovation as you describe but to 'handle' the needed space in another way.
And lastly, as to who to trust, you're going to have to ask around. Ask for references, ask the BBB, see that the Contractor is insured AND bonded. Get everything in writing and DO talk about how 'Change Orders' are handled. Talk about whether or not you'll need to be OUT of the house.
I'm sure there are good books in the library and/or bookstores that describe the Home Remodeling 'process' in more detail. Maybe a good book is worth some of your $$$ too. Perhaps others have more to offer. I do wish you well.