As an RV owner, a builder, and having extensive experience as an instructor, I can tell you your goals are not mutually exclusive. You may start building and love it, or lose interest, and the same is true of flying. Either way, no harm done, kits and tools can be resold and you'll still be richer for the flight training experience should you decide to walk away. It IS quite a lot to take on at once, both financially and in terms of time to devote to the projects. A majority of the people I have trained who were working a full time job and had family or other obligations struggled just to find the time to study and fly often enough. Depending on where you live, weather may slow down your progress. Many student starts drop out eventually because they underestimate the time and money involved, or because they realize it just isn't what they expected.
The same is true of building; it's quite easy to be starry-eyed and motivated at the beginning. We finished our empennage in less than 6 weeks, and thought "wow, this is so easy!" Six years later... But we're still plugging along and I have nothing but admiration for all the amazing builders here because I now understand the scope of the job they've accomplished. It takes an exceptional person to commit to and complete either endeavor.
Nevertheless, what you're contemplating can of course be done, it will just likely be neither as quick nor as inexpensive as you might imagine. Either would be rewarding, so I would say go for it. Just be realistic about your expectations, and be prepared to sideline your build to focus on your training because the kit will quietly keep, but flying is an acquired skill that requires repetition and frequency to maintain.