Sure, here are some basics:
(i ) Material - The most common base material is crushed limestone. Referring to it as "A-2 base" means that you specifically want crushed limestone flexible base that meets TxDOT standards for aggregate size, liquid limit, and plasticity index. In a nutshell, that means it will perform well.
(ii ) Thickness - This depends on two considerations - load capacity and soil conditions. The thinnest profile might be 2-3" of base if you are considering passenger vehicles for a driveway constructed over weathered limestone or caliche. The other end of the spectrum would be a 12" base profile for a fire lane/heavy truck drive lane over fatty clay soils. You asked about heavy vehicles and ag eqpt so that would be likely 6-8" minimum, more likely 10".
(iii ) Subgrade preparation - All organic materials should be stripped and removed. That means grass, vegetation, and the top 6" of topsoil. Ideally, you would scarify, water, and compact the next 6" of material, but that would be overkill for a non-commercial/industrial project.
Also, the base material should be installed in 4-6" lifts, watered heavily, then compacted with a ride-on roller - preferably a pad foot finished off with a smooth drum. Properly installed flex base will seem like softish concrete when installed properly, after it dries.
