For a dory of the 1750's running bowsprits and centerboards even are overkill or anacronistic. You don't see centerboards until after the Civil War..... For a rig a sprit sail would be OK if you have to have one. In the 1750's rowing would have been the usual way to move about.
Gardner has some interesting stuff on the big dories aka batteaus in The Dory Book. If you do need a bowsprit, the simplest one is the one the old fishermen used...... a bowsprit with a slot that fit over the stem head, and a pin through the stem head to keep it in place. In your boat then a lashing aft to keep the heel down as you have no foredeck, and lashings to the open rails on each side to keep side by side movement down. Depending on where you mast ends up, you could also run the bowsprit back and lash the heel to the mast or set it up like a boom with a couple of ears to keep it from moving. You should not need a bob stay. The mast should be unstayed.
Double thole pins was the custom in dories. The single thole pin and grommet is also old, of your period, but seems common in Scandavia. As far as your thwarts go ( looking at your earlier post) you should need no more than 30 inches between rowing stations if people are in synch.
And for armament boats this size would depend on mustkets. A "rail gun" would be the biggest that might get mounted if you went with something fixed, and there is no evidence for stuff like this in an 18' boat.
Have fun.
Ben Fuller
Ran Tan, Leste Kuhling, Vernon Langille, Josef W., Merry Mouth, Imp, Macavity and a quiver of unamed 'yaks.
"Bound fast is boatless man."