Forestry does not require buildcraft to play and there are many mods that have buildcraft energynet integration.
To be honest I do believe that You can play only forestry and be happy with it. Forestry do also provide own buildcraft engine, so there is no problem in powering your machines. If you have installed also railcraft or thermal expansion or Mekanism or Power Converters or similiar BC compatible mod, you can freely skip buildcraft.
Even using a forestry engines it is possible to have some fun with forestry. Cause those engines do run for a quite time when provided with fuel.
Wiki tells us that one peat in peat engine do run for 5000 ticks and that is 4,16 seconds, so whole stack will run for 262.4 seconds (that is 4.37 minutes), and full engine that can hold 4 four stacks of peat do run for 1049.6 seconds, and that is 17.49 minutes. That isn't really bad. When we also add hoppers... Well you could make a hell of a run. And that is one of the basic engines.
Also most of forestry machines don't eat out a lot of energy, so we don't need top tier engines from other mods.
Also forestry do have it's own world gen, so every raw resource that is used in forestry is also provided by forestry.
Also none of the forestry machine does require to be operated by other mods machines.
Also forestry does even give ability to transport not whole buckets of liquids, by item called pipet.
But to sum up. Buildcraft is not required to be happy with forestry.