135 and 142 are actually the same thing in terms of the space between the dropout faces . 135 is the over lock nut distance for a QR and some early thru axle designs which had the hub clamped directly to the dropout face. The 142 hub just has a 7mm wider over lock nut distance to index the wheel in a 3.5mm recess in the dropout(135+3.5+3.5 =142). This makes it easier to quickly install the wheel and insert the thru axle. Early 135 thru axle bikes did not have the recess in the dropouts and you had to fiddle around to get the axle lined up with the hub. In any case the distance from the dropout face to the disc is the same for any hub width.

Both of those dropouts are for QR wheels and I am not aware of any 142mm QR hubs. 142 is simply the thru axle version of a 135mm QR hub.

In reference to the types of brake mounts, low mount is for post mount brakes. There is no easy way to make a flat mount brake work on those dropouts. Shimano and SRAM are moving toward flat mount for all drop bar disc brakes, so if you are building a drop bar bike, I would go with the flat mount. Post mount is still being made for MTB applications.