Sorry for taking so long to reply, I just noticed this thread.
For me, the deciding factor is how are you going to finish off the building? Are you going to pour concrete? What type of ceiling do you want and how big of a span do you want?
The most affordable building is a pole barn where the posts are set in the ground and purlins are run from the post and trusses across the top. The wider the span for the trusses, the more money you spend. There is a point where metal becomes more affordable then wood trusses on large spans, and then there is the added benfit of the open arch of the metal for a roof wich is especially useful for a commercial building.
When you add concrete to a pole barn, going with posts now becomes an option depending on how you plan to finish off the walls. If you want just a shell on the inside, then posts are the most affordable. If you want finished walls, like plywood or OSB or even sheetrock, then posts are not longer the best option since you will be framing up the walls for the interior siding. Then stick built becomes the better option.
In my opinion, 12 foot walls with 2x6's stick framed on a concrete slab is the starting point when figuring out what to build.
For a roof, if you go metal, you can get away with a lower pitch, which may or may not be an issue. 4:12 is what I like and what I'm comfortable walking on in dry conditions. Steeper has a nicer look to it, but it's gets a lot harder to walk on if you ever have to go up there. For shingles, 4:12 is the minumum slope and 6:12 is about the steepest I'm comfortable walking on. Shingles are going to cost more money in trusses because of their weight. You have to put those trusses on every 2 feet and then put decking over them. For metal, I like my trusses every 4 feet so I can put 2x4 purlins on the flat on top of the trusses for the metal. You can go a lot farther apart on your truses, but then your purlins have to be on the edge. This gets to be a math challenge if you want to cut down on trusses and increase the size of your purlins that is beyond me on where the ideal span is. I just stick with trusses on 4 feet.
An added benifit to 4 foot spacing of trusses is that you can easily install metal R panels for your ceiling on the inside of the building without adding any additional lumber. If your trusses are farther apart then 4 feet, then you will have to build a frame work to attach a ceiling. If you want plywood or sheetrock for your celing, then 2 feet is the fartherst span you can get away with. and if you use sheetrock, it has to be 5/8's or it will bow on you between joists.
The thing that I like about metal on the ceiling is that you can blow in insulation in the attic space yourself. Comparete to all other insualtion options out there, nothing else comes close to what it costs for R value up there. Foam is better, but a lot more money and a very long return on your investment when compared to blown in. If you are going with Metal, or an open ceiling, then spraying closed cell uder the roof metal or shingles is the best choice. Never used open cell, it holds moisture that comes from humidity and leads to rust or rot over time. Open cell is cheaper and easier to install, but in the long term, not a good choice.
Door size, lean to and layout are all more depending on your use and location. I would start the process on where you want to building to be, how you will access it and if you can add on to it in the future. Bigger is always better, but that's more money. Building the main structure now with space to add leantoes onto the sides in the future is a good way to get your building done now, and still have the ability to have more space if you ever need it.
Hope this helps,
Eddie