As Andrew said, benchwork is not hard to do. Home centers like Home Depot, Lowes, etc will do one cut on a piece of luymber for free. If you buy your 4 x 8 plywood sheets and have them cut in half to make 2x8, you will have ready made benchwork. Just get some 1x2 or 2x2 for legs and cut to the length you need for the correct height. Then get some 1x2 to use for bracing. Just remember that the strongest and lightest structural member is a triangle. If each of your legs has a 1x2 running across horizontally connecting the two legs at the top and near the bottom, and one running diagonally from the bottom of one leg to the top of the other, you have two triangles. Run one 1x2 8 feet long down the center of each 2x8 ply on the bottom side and then run a diagonal brace from the bottom rung of the installed leg to the center beam, and your benchwork will be solid. Space the legs the same width as the plywood top, and then nail a 6 inch masonite strip the full length of the plywood fastened to the side of the legs and with small blocks of 1x2 pieces fastened to the masonite and the bottom of the plywood every 2 feet or so as necessary for strength and you should have nice strong benchwork with a fascia board to build on. You might want to have one or two of the 4x8 plywood sheets cut at a 30 inch width for the side where the yard is located. If you don't need to have any other section of the layout at an 18 inch width, cut up the left over plywood into 4 inch wide strips to use for the cross bracing on the leg sets. The main thing to remember when dealing with plywood is that screws and nails hold very well when going across the plys, but plywood does not take fasteners very well between the plys, it tends to delaminate.
Regarding the track plan, it is a nice start, but the two mainline runs seem to be only connected by the yard, and the double ended yard ladder will minimize the number of cars the yard will hold. Also there is no yard lead so that you can work the yard without fowling the mainline. I would eliminate the mainline coming off the inside of the yard. Then I would bring a yard lead off the mainline near where the drawing is labled "industrial" and bring that lead all the way across to the other side of the layout just below the roundhouse and turntable. Run a lead from the turntable next to the roundhouse to connect to the lead by a turnout. Now you have a yard lead and a path for your hostlers to get a mainline locomotive out of the engine facility to the train that the switcher has made up. Run a double ended siding off the lead so that a train could go to the main in either direction, and then run a stub end yard ladder off the siding. Keep the mainline to the back of the bench work so you don't have to reach over the main to work the yard. You can use the double ended siding for a run around track and make your trains on the lead. You can now run a single track mainline with passing sidings around the room. Plan your industries, mine, town etc. and install sidings as required to work those industries. If you are going to do multiple operators/trains, I would tend to put the industries and sidings toward the inside of the layout and move the mainline more toward the rear again to avoid having to reach over the mainline to work the industries.