Go with the 1850, I don't think you will regret the HP & you can always throttle down.
The Post Driver hooks up to everything, PTO IN & OUT, Q/A, and the drain line. First you set plumb from the tractor seat, then you square it up L/R with Q/A, then you set the parking brake, turn on the PTO, and stand next to the driver to operate the drive lever. Opening the drive lever raises the dead weight, and releasing the lever drops the weight. While Terry says that you can't lower the weight without driving a post, you can crack the valve and ease it down, but by default the valve lifts, and the moment it is released the weight comes dropping down- So watch out for fingers and toes. Basically, you are raising a 6-800lb weight up and dropping it on the post.
It is really important that you square up the driver, or you will get an out of square post. (As Homer Simpson says "Doh!") It is not hard, but do take a level out with you.
I use a heavy duty bungee cord to hold the post against the driver, with the hooks in front to roll along the post. It helps keep the post from shifting, and it keeps my hands well away from the post while driving. The only downside to the driver is that when people see how easy it is, they will want you to do lots. (So don't let your customer's see you operate it!)
If you are planning on do a lot of landscaping, you might want to consider a laser control on the power rake. It will get you to level or to grade rapidly.
Many attachments don't require the use of the Q/A hydraulics.
Have fun,
Peter