Standard pole barn screws...they have a rubber washer on them to serve as a seal and more importantly to "snug" the panel without wrecking it. Not cheap (but what is)? Available in several colors. Your metal supplier should have them but basic colors are available at Menards so I assume all the big box stores have them and your local hardware store might also. I recall they are a 1/4" hex head but check. I think a layer of poly sheeting is important. Prior to install you can choose to "block" the soffits with suitable material if your building has a ridge vent and perforated soffits (they serve as your ventilation system to carry out excess heat and moisture). You don't want to fill them with insulation. I think corrugated cardboard blocking is available but I think I heard that it will disintegrate with time. As noted previously I opted to form a "bank" around the perimeter with thick fiberglass batts (2 layers that got to R-50) then used blown-in material to fill in the field also to R50. My 30x40 heated space cost $1,000 (hired out) to insulate. You'll want to have some way to "navigate" above your new ceiling so plan ahead there (as noted previously I had 2X4's strung 10' apart on top the bottom cord of the trusses...I know you aren't supposed to place too much weight on them but if careful you'll only stress them once (insulation install) and then you'll have your nailers for lighting and lightweight hanging stuff. I do believe that you are going to love your finished product. It will look nice and you'll appreciate the light reflection. Good luck! Btw, I promised a picture but my phone camera didn't seem to capture what it looks like. I'll try again with a different camera.
I thought pole barn screws were expensive! Almost free in comparison to stainless screws aren't they? And I don't understand the "countersink"...the metal (liner or thicker building) is not that thick. What can possibly be holding up your metal if you countersink it? I think I'd stick with the standard pole-barn screws...that rubber washer is there for a reason.
That "dent" method I learned from my builder worked great...stack several sheets, measure the top sheet (rafter spacing along the length and next to each major rib on the width) and give it a "whack" with a center punch. Those dents show the screw locations and you don't have chalk lines or a lot of pencil marks to clean-up. Probably not a big problem on the interior but drilling was a "no-no" because it leads to premature rusting. When the dents start to get faint, insert new sheets under the pile and give it another series of whacks. Ceiling work is definitely a 2 person job and scaffolding would be a big help as would "dean man" supports. Panels aren't heavy but they can be awkward to handle (you don't want to buckle them). I let my builder to it and he flew through it using a homemade scaffold that OSHA would have had a fit over but he had been doing pole-barns for decades.
