To quote some people that really know what they're talking about:

Fit and fitness go together.

Core strength [or lack there of] is an important factor in fit if you're looking for the non-bar stool position down the road. The stronger your core is the more 'aggressive' your position can be [comfortably I might add] and the more power you'll be able to generate in that position.

My coach advised against getting a custom frame until I had a solid year of training back on my legs/body. And that year was coming back to training after being a Cat III roadie years ago and living a very athletic life style since then.

Of course [again] it depends on your goals.

Your mileage my vary.