Your’e question spans a lot of subjects while leaving a lot of unknowns.Your best bet will probably to find a published track plan that fits your available space. Keep in mind the area around the layout is where you will have to be to work on it and where your visitors will be observing it. In other words, don’t try and squeeze the largest plan you can find into your room.
As far as having mutiple levels of track and scenery, a lot of available track plans have the benchwork requirements as far as materials and special cutting patterns and dimentions. By choosing a published plan, by default you have let someone with more experience in the hobby do all the hard parts, making it more enjoyable for you. You will see as you go throgh the construction why the plan/benchwork was designed as it was.
A solid start will be back issues of Model Railroader magazine. Ask your local Hobby Shop, search online, and Atlas has a wide variety of track plan books.The Model Railroader magazine plans range from very simple to vey complex. Every year, they start a new layout, and go through the construction process over the next couple months.
Don’t be intimidated by starting construction, but, don’t bite off more than you can chew. The fun will be learning new techniques. Don’t be afraid to ask!
Without seeing your available space firsthand, it’s hard to determine what exactly will fit your bill.You will be the best judge of that.
Personally, I wouldn’t start building until I have a plan. You say you are going to use access holes. How do you know where you are going to put them until you have the plan. As soon as you place your access holes you limit what you can plan. Look in my signature for the beginners guide. Unless you have a lot of money to waste making mistakes, I’d suggest slowing down and thinking things through.
I do believe he said he was using a ready made Atlas plan.
Anyway, to make the various grades, you can use the “cookie cutter” method. You use sheets of plywood or foam for your layout, then you lay out the track to make sure everything will fit properly. Then mark the track lines so you know where it will go, then determine where you want the track to go up or down. Mark lines about 1 1/2" from the center of the tracks (about 3" wide total) where you will cut the plywood or foam, then cut it using a jigsaw (for plywood) or sharp knife (for foam). Leave the track boards in one piece, don’t cut them if you don’t have to. You can gently bend the wood/foam up or down to create hills and valleys. Doing this will give nice smooth, curved transitions from flat to grade, you don’t want the track to suddenly go from flat to hill. Try not to use more than about 2% or 3% grades if possible, which is 2" or 3" per 8 feet, respectively (actually 8’4", but it’s close enough). Use scrap pieces of wood to support the track boards. You can use scrap pieces of 1x4 wood to join the edges where boards have to meet, so the boards have a nice smooth joint instead of a hump or dip. If using foam, use scrap pieces of foam glued (with foam friendly glue, of course) for joints. You can use the non-track parts of your boards cut off and raised up or lowered to build towns, plains, mines, rivers, lakes, or whatever.