The Mech Shop also gives access to the 'Build your own' Constructed mechs interface. At level 55 the link for Mech Construction appears at the top of the page.
Mech Generation Edit
The Generation number of a Mech (often called Gen30, for example, for Generation 30 Mechs) is the minimum Commander Level needed to be able to buy and control the Mech. Note that the Generation shown will obviously prevent you from buying it until you reach the required level, but you are still allowed to preview base stats of all the mechs available to purchase.
A single Generation may have multiple models (for example, Gen30 has both the Pseudo-Aversion and Pseudo-Inversion Mech models).
Mechs above gen30 are called leader mechs, they have additional group bonuses that are applied to the whole squad. Up until lvl 54 all Mech's have a skill cap of 100k.
Tier 75 Mech's have a skill cap of 115k.
Tier 90 Mech's have a skill cap of 125k.
Tier 100 Mech's have a total skill cap of 120k.