Hey Star Trek, I have empathy for your situation, it appears to you are in rock and hard place, and, it is difficult to leave a cult completely behind. I sympathize with you. We are all damaged to some degree.
Moving on to business:
Personally, If I didn't have the cash, I wouldn't try to buy or build a house unless I had a banks approval first with an agreed upon period of time to close. Working with a builders' financier's in an expanding economy is not the side of the table where you want to negotiate from. Get your financing lined up, i.e. make loan applications to lenders, then go to a neighborhood that you like and negotiate a price for a home.
If the new prices are outrageous in a hot area and you can't afford it, then perhaps look into a used home that you can revamp. However, if you walk in to a builder, coolly look at their models, say, "I don't know, I'm not fond of some of your selections", and you have a letter in your coat pocket that says's you have a loan from a such and such bank for six months to 95 percent of completion, you can then negotiate a good price for the house. I wouldn't play by their rules. (This is a whole other topic for a thread)
What you are doing is comparable to going to an auto dealership and letting them ask you what payment you can afford as opposed to what car you want. There is a world of difference.
Read the Donald Trump's "The Art of the Deal" (used, 50 Cents). There is nothing superbly outstanding or moving the earth in it, it just gives an insight that some may not have naturally.
Best wishes. And.....yes, I would tell your MIL your moving. Let her make her choice.