Regarding Construction
The architecture system isn't entirely free-form, instead it is a combination of blueprinting (a skill tree) and known styles (in game skill knowledge). As a result there are mechanical limits on the type and shape of structures that can be designed.
Their is intent to release the blueprinting interface before launch though a time table hasn't been given.
Regarding Leveling Caps
It is incredibly unlikely for anyone to achieve legendary level in any craft in a single lifetime. Instead the goal is to amass experience in a craft over multiple lifetimes.
As an analogy, every time you reincarnate you effectively prestige your skills (reset to 1st level). However, as a reward for prestige-"ing" (ala a COD shooter) you are given an on ramp. This on ramp increases the rate at which you gain experience in the skill you had previously, and compounds across lifetimes as long as you maintain training that skill.
In essence after a couple lifetimes your soul turns whatever body it inhabits into a natural born prodigy in whatever skill you have trained. The prodigy still needs to learn the skill but does so extremely quickly.
As far as caps are concerned, the engine has some target goals regarding the amount of the population in each of the "tiers" of skill knowledge. As a result, the difficulty in gaining experience is dynamic. This acts as both a soft ceiling and a soft floor.
I hope this helps.