My shop, 24X32 used to be a drive in photo finishing store. It was going to be torn down so a guy got permission to tear it down and save it. Then, when he moved from Duluth, he gave it to another guy that he owed money to.. It had laid in a pile in this guys yard for years and that is where I came in. I went and looked at it and started tearing structural pieces out and trying to figure what they were and where they went. I had no experience with steel buildings. Anyway, I bought the pile for $900 and it laid in a pile at my house for a few years. Then, I had a slab poured and I put the whole building up by myself. And it's great having a shop.
I think building one yourself is a mistake. It would cost you more than a few times more than it would to buy one. Keep your ear to the ground, you will find a deal sooner or later.