More Dumb Newbie Benroy Questions

While I am waiting on Red Trailer to ship me the correct rear rail for my trailer, I decided to take the plunge and start buying the lumber for the floor and walls today. I was all excited and almost skipped into Lowes with excitement. I had made a list of Mike's Generic Benroy Plans of what I needed to build the floor. I went and picked up the 1 1/4" Stainless Steel Screws-Check, Then got the big container of Gorilla Glue-Check, then went to look at the 1x2 lumber. I am one of those guys that needs to be told every detail so i stood there wondering if I was supposed to get 2x4s and rip them in half. I got confused standing there looking at all the crooked and picked over lumber. Nothing was straight. I headed down the "Fancy Lumber" asile where they keep the expensive Oak and Maple lumber. I found a section of 1x2x8 pIeces of Select Oak for $7.03 per board. I thought it was high but figured I wanted the best lumber I could get. It was straight as an arrow and excellent looking so I grabbed 8 of them. (I hope this will do)...Now the quandry, I went to look for 5'x5'x1/2 Baltic Birch only to find that Lowes and Home Depot neither carry that size nor will they order it since it isn't in their special order books. I essentially drove around for 4 hours to 6 different Home Depot, Lowes, & Ace with no results. I got frustrated and finally spoke with the Lumber Dept Manager at Lowes and he said in 30 years, he never ordered a 5x5 birch stock. not wanting the day to be a total waste, I decided to go ahead and buy the plywood for the walls. I ended upi getting two pieces of 4'x8'x3/4" Birch SelectFurniture Grade Plywood at $39 per sheet...I hope this the right stuff for the walls???
Some of you pros need to reassure me I can do this. I was about ready to chunk the lumber and just spend $5000 on a Little Guy. Where can I get 5'x5' Baltic Birch for the floor? Also on the 1x2 stock for the floor, do I lay it flat end down to butt it together or skinny end down? Make sense? It's been a long day and I need HELP!!!
Some of you pros need to reassure me I can do this. I was about ready to chunk the lumber and just spend $5000 on a Little Guy. Where can I get 5'x5' Baltic Birch for the floor? Also on the 1x2 stock for the floor, do I lay it flat end down to butt it together or skinny end down? Make sense? It's been a long day and I need HELP!!!