What a great thread to read! I am a new Mooney owner, and am still a student pilot currently. I would like to give my thoughts on this, albeit very limited still compared to the rest of you!!
So, I am about 54 hours in to my training, and 48 of those were in a C172. While, I believe that training in a complex from the get go is doable, I can't imagine how much more involved it would be. The Mooney sorta fell in to my lap, as the other GA guys on field where I train knew I was looking for one. This one was located on field, and the owners couldn't fly it anymore and needed to get rid of it. Anyways, Going from the 172 to this was AMAZING. The differences were way more than I expected, and am extremely happy that I have the knowledge, training, and hours on the 172 prior to the transition. My instructor and I sat down, and discussed at length the advantages and disadvantages of finishing my ppl in my Mooney, but in the end we both felt that I was ready, and decided to transition.
Learning everything, from communications, to maneuvers is not crazy different, but enough that having the ability to go slower, and not have to be perfectly on the numbers on every landing, etc has helped lesson that learning curve of starting in a complex.
With that said. I can only take lessons twice a week, which gives me a couple hours of flight time a week after the ground work. Having more time to dedicate to this may have changed my opinion on this. At the end of the day, I am thankful for starting out on the 172, and then moving to the Mooney. It also gave me WAY more respect for my instructor and his constant push for me to focus on pattern work, and speeds which has helped me 100%.
For insurance. Everybody is right. Uhm, I don't want to discuss insurance lol. There is one company in the country that will insure a student pilot with retractable gear.. And they make you pay for it. HOLY CRAP!
Good luck with your decision!
-Chris