What I found handy for aluminum fab was two get 6 in flat bar rather than plate,
if you can buy small quantity, always pieces on the net.
2 feet each of 1/4 in. 3/8 in, and maybe 5/16 will get you thru most project stuff.. not sure about rear brake bracket, fitting on 6in.
If you are adapting stuff like brake calipers on the front, you will need thicker but milling is also usually involved.
6061-T6 is what you want. You can cut most stuff out on a small bandsaw if 6 inches wide, but plate will get hung up, then you need a skilsaw.