The cheapest way to automate the moistener and farm is to us stone/cobblestone pipes and a couple diamond pipes.
If you can make gates there is an easy way to make sure it never overfills, but you can just split a safe amount of wheat using diamond pipes.
Example:
Overall farm:
Power the farm+combine using an MJ producing engine of some kind.
The diamond pipe over the farm place a seed in black. This will fill the farm first with any seeds and left overs will continue on. All wheat will continue on automatically.
Go a block or two and put another diamond pipe down. Connect a chest to it and pipe downward/toward a moisener (i normally bury these so they are dark).
On the chest side (yellow in this shot) put a little more wheat than in the black down slot. If you ever get your moistener overfill then adjust the number of wheat in the slots. You can get very precise with it if needed. Like 3 black and 7 yellow should should give the chest 70% and moisener 30% of the wheat.
Do the same for seeds (50/50) split shown in image above.
Down at the moistener split the seeds off to input at the top and let the wheat continue on to a side input.
On the right side of the machine attach a wooden transport pipe and a redstone engine to pull out the mycelium/mossy cobble and mulch:
If you want mossy cobblestone the easiest method is to remove seeds from going down (let the extra seeds go in a chest at the top diamond pipe). Remove the lower diamond pipe and input pipe that goes to the top of the moistener, so you just have wheat going into the side. Put a thermal expansion igneous extruder on top of the moisener - set to cobblestone and output out the bottom. It will keep the moisener filler with no power requirement.
In the shot above water from an Aqueous accumulator is pumped into the back side of the moistener -- the bottom side is free as well and will accept liquid.