I was a little luckier in my timing when I started my RE investing career. It was the early 2000's and there were ton's of houses that just needed lots of cosmetic work. As the RE market picked up headed towards 2007/2008 (and the TV shows started coming out) the deals were harder and I had to cut my teeth doing houses that were increasingly more difficult. During those intervening years, I bought a big building to use as my shop, started a GC business and hired a full time crew. That was all before I ever did my first gut and remodel.
Doing one out of the gate is a big job. You will get there, take your time, visit the site very regularly and make very concrete plans for how you want things to be in the end. Making changes half way through will be costly. Don't be scared to "offend" someone that is working for you. Buy books on how to do things like plumbing, and wiring, don't rely on youtube videos. You don't have to do the work, but you should be able to recognize if someone you're paying is taking a lot of shortcuts.
Some of the mistakes I have made over the years:
Replaced the sewer line and cheaped out on replacing the water line at the same time. Ended up having to dig up the brand new sidewalk.
Installed plumbing vents through the roof and didn't know how to properly tie that all into the roof. Sucks to rip out all your new drywall.
Bought a house and after gutting it discovered that the 2 story addition was falling off....in the middle of winter and a snowstorm.
Had my copper stolen by a neighbors kid that was a junky....3 times.
Hung and finished all the drywall in a house before I practiced like crazy and got good at it. It came out absolutely horrible. Used so much joint compound the cabinets didn't fit when it was all done. Plus I think it took me about a month to do it.
Not knowing what a banks underwriter will look for when inspecting a house. Having to rip open the ceiling in a brand new bathroom to install a vent fan before settlement stinks.
Cheaping out on windows..if in doubt...just replace them. It's a HUGE pain after everything is finished.
Installing jaccuzi tubs, total waste of money.
Sprayed a house before installing windows. Sounded like a good idea, until I had to pay to get a neighbors car detailed.
I'm sure the list goes on. Actually there should be a thread on rehab mistakes.