Jeff should also have some comments on what your Rehab might take. It's great that you have a "Pro" in the mix, but I am telling you, I RUN OVER from time to time, and I've been doing this for a decade. I had one this summer that ballooned to 300% of budget. It happens.
In your case, the Wife is going to want the High end stove, where the buddy was thinking a mid range, and it gets away from you starting there.
One of the MOST expensive things that can happen to you is changing plans. You get everything set with the Electrician. After the fact, your Wife (Most likely the wife

) makes a great point that the overhead ceiling fans should have separate controls for the light and the fan so you don't have to pull the cords a thousand times to get the fan right. Everyone hates that, separate controls is the ONLY way to go, you just put it on High, and have a 3 setting switch on the wall built into the same light switch. However, this requires another wire run. No big deal you think.
THIS IS WHERE the Electrician just rapes you. Piles on, and goes nuts. Anything you change costs you a mint. All these contractors know that you are going to change things. This is where the New Rehabber gets eaten alive. They don't know what they want, and Changes happen at 4 to 5 times the going rate.
Trust me, your Wife (and You) are going to change things as you go along in your dream house. Who wouldn't??? I would, I'd change everything. This is my Dream house, and if I see something in the construction that could be better,. I am changing it.
All I am saying here is that this isn't your buddy's normal flip. This is your dream house, and YOU and YOUR WIFE are going to really run the price up here.
Did your Buddy allow for the top top end Faucets in the Bath? He didn't in his figure. He doesn't use those. Your Wife is going to want them, remember, this is her dream house.
This is going to get away from you. I do this for a living, and if I was standing right beside you, doing everything I could to help, well, then it would get away from US.

It's just the way it is going to work. The Carpet your Buddy uses that is good enough won't be good enough for your home. The list of things you are going to go over his normal quality on is about Everything in the house. It's just the way it works.
I am off on a Tangent here, but the point is that you are not leaving enough of a Safety net, and that is coming from a guy that is a "bit more conservative than other 'rehabbers' "
So back to the Co-GC deal. You need to follow his vision, and accept less than perfect, or he needs to accept yours and up the budget. If you each run for 3 weeks at a time, you will not be happy with the product unless you buy into what he is selling. Someday I will tell you about my first and only partner. This was exactly our issue.