I decided to look at sourcing the wood for the cab today. There were two places I considered – Bunnings and Melboards (located in Hallam – so close to me). Bunnings won out. I estimate I will need 3 boards of 1800 x 1000 x 18mm. Bunnings has 2400 x 1200 x 18mm for $33 each. This should leave plenty of left overs for things like the base, top and front panels. I suspect I will need some additional wood for bracing and load bearing, but that will be purchased separately, and cost much less.
Thickness was a decision I made based on some advice and general research. Anything below 16mm will be too thin, especially for an upright. You could maybe get away with 12mm for a counter top, or a very cheap table top. The real choice is between 16, 18 and 21mm. The good thing about 18 and 21mm is T moulding is easily obtained for these thicknesses. If you go with 16, you will need to do additional trimming, or try and source smaller T moulding. I felt 21mm would be too thick for my cab, so 18mm it is. I guess generally speaking, the bigger the cab, the thicker you should go to ensure it’s sturdy.