The race you choose is all up to you. Personnally, I chose a Nolthrir (elf) because they more inclined towards magic than many other races, due to the intelligence they start off with. I find intelligence more important than strength, although it can help sometimes.
As for the limitations, in the Player Guide, it is said that the future rules for the Klyros, for example, will prevent them from wearing heavy armor, as it will render them unable to fly (another rule for them).
As for the Kran, these limitations are based on the race's physical limitations- Nolthrirs will be able to breathe underwater, because they spend quite a lot of time in the water, and so they have evolved to conform to that.