Completely RAW, Balanced, and Unconventional Races (Spheres)

[SangoProduction]

So, for context (I haven't really delved into Spheres), how much build resources does this take to get?

And what sort of limits are placed on what you can select? Is there any cap on base stats you can have with your new form, how many limbs or attacks it gets, or what racial abilities you can get with it?

http://spheresofpower.wikidot.com/alteration#toc41
It takes just one feat, the Transformation feat. You get to select any of the (transformation) talents. The limbs and speed are define, as well as any natural armor or weapons. You also gain any abilities under Special, given that it's not saying you spend spell points.
You get to select no "traits" unless you pick up Improved Transformation (Or choose Anthropomorphic Transformation, since that's part of their Special, designation). And some of the forms don't grant speech (which is not a thing in the 5e version - they realized their mistake), which is probably going to be a problem unless you take Hybrid Transformation to "retain" those capabilities from your base human form.
You do not get additional stat points. (Exception to if you get the Size Change trait, which grants +/- 2 to strength / dexterity, depending on whether you go small or large).

To emulate additional abilities that are iconic beyond the base aesthetic form requires slightly more effort. And is sometimes functionally impossible within the context of purely using Spheres - like dragons getting racial vancian casting - but you can just... take this feat, and go normal sorcerer to emulate that, and the Behir has 12 legs, which is functionally impossible to explicitly replicate but does it have functionality of more than 6 legs in any realistic situation? Probably not.

But the biggest deal is that you are aesthetically any creature, and if you're replicating a powerful type of creature, you put more build resources into becoming more like them as you level up. But you aren't "trapped" in racial levels. And you don't "have" to take every mostly worthless racial trait of a given creature, like Savage Species does with like 50 more pages of rules.