Ok if you want top shelf find this stuff
http://www.fabral.com/product.php?id=22
This stuff aint bad either
http://www.abcmetalroofing.com/panels_imperialrib.html
If you sheath the roof (not needed for metal roof) use this stuff between the roof deck and the metal
http://store.guttersdirect.com/rotopguiiun5.html
Get and read the installation instructions you do not know how to install it so don't even think you do until you have read the instructions. Don't second guess the instructions they are right and they are right for a reason.
All those other other trim parts serve a purpose use them. Mastic was made for a reason that reason is to be used on your roof.
Yes 2 foot on center is adequate spacing for the purlings on your new roof. No 1x's are not thick enough for purlings.
Put your purlings on with a string so that when you drill the panels for the screws they actually hit the purlings all the way down the roof.
No don't just stop at the borg and buy panels that are to long. They cost the same or less to have them made to fit just what you need.
Biggest mistakes I catch driving by a new metal roof
1 Not screwed right (says right there in the instructions how and where) most people can't read.
2 Panels ordered too short ( can see the splice that was added on to get it under the ridge.
3 Panels lapped backwards. They only go one way and they lap that way for a reason.
4 No trim - no trim means the rated wind load has not been achieved.
5 Screwed in the high rib. To date Morton is the only roof that is designed to be screwed in the high rib that I have seen.
6 Roofing cut with a saw - never cut roofing with a saw.
7 Saw toothed roofing install. When the ends of the panels don't line up.