we just priced out planing for 8x8's at $50/1000 board feet here in ontario....
we figured there would be another $50 for set up and at 2-300 for trucking the timber
another option are the hand held power planers from places like makita or mafell(expensive)
If you are interested in the planing mill option i would suggest calling some local sawmills, they will know where the planing mill are.
another option would be adzing or broadaxing the finish for your beams....you could power plane the smaller pieces and adze or axe the main timbers, this is a common finish on the older barns in our area.
as for drilling, you can place lines on the ends of your timbers that are parallel to your mortises, then screw a 3-4foot long straight board that hangs out over the sides of the end....this will guide your drill bits in free hand use, we are using this method with round timber right now and the mortise sides are coming out fine
or look for an old boring machine, these drill acurately and are great excersize too....
have you seen a electric chain mortise machine? i saw one for the first time recently and for square timbers i think the expense of these machines would be easily justified
good luck
-Mark in Canada.