I would like to build a structure in my backyard to use as a home gym -- a stationary bike, treadmill & a few weights. I'd like it to feel somewhat livable & more of an extension to the home, rather than just a bare enclosure.

Has anyone looked at any of the prefab shed / studio options out there? Any thoughts on how to evaluate one vs. another? Here are a couple I've found:

MetroSHED
Modern-Shed
Kanga Modern Studio

I would build one myself but I've never done something like this before, and would like something a bit more pre-fabricated to help me along.