Oh dear, oh dear sooo much information soo little time.
Just to start off I have been racing for about 5 years and manage an outdoor 1/8th scale track in woodland, WA. Myself and a buddy (and a tractor) built the entire track which measures 140x130 feet. I have also built two 1/18th scale tracks. A few thoughts:
* Sod cutters are very helpfull to get you started but it seems your halfway there.
* You could topdress a 1/18th scale track with clay. You will need about a dumptruck load or so (about 7 cubic yards). Call excavation contractors in your area. Clean fill dirt (from a job site that needs to get rid of it is about $4-5 yard delivered).
* the clay needn't be deep maybe an inch or so..the reason is that 1/18th don't have the weight and power to tear up the track. I should note, the above post about not mixing the clay in is TOTALLY wrong for 1/10th and larger. A layer of clay ontop of silt or sand will break apart in dinnerplates in the summer unless it is at least 5" deep. Not cool
* I see you have drainage pipe which works fine. For 1/18th I use 3/4" pvc or abs. Effective, cheap and easy to use.
* If you get new dirt or wan tto build a new track, scrape the ENTIRE area..not just lanes. Add dirt and rake smooth, and rake, and then rake some more.
* use some paint to mark where the lanes will be. Plan to use the pipe to make chicanes and such. Avoid 180' turns, especially at the end of straight aways.
* Then take a yard roller and roll everything flat. Fill i the depression or they will fill with water. When your done rolling. Soak the entire track. Soak it very well in the early morning.
* Now here is the pimp tip, don't!!! roll the wet ground until the very top 1/4-1/2" is dry. The top will dry out a little and the bottom will still be damp so it will compact well. If you get ansy and roll the track when it is wet, the dirt will stick to the roller and you'll be back to cussing and raking.
* Let it dry out for a day before walking all over it, lay out your pipe and your good to go
Other random track tips:
* Sugar added to the soil will make it hold moisture a little longer and also serve to make the dirt a little more cohesive.
* If dirt is hard to find the bigger tracks can use haybales covered in dirt for topo. Try to find "damaged" or rotting hay they go for about $2 a bale. If you cover them well they will last for 2-3 years.
* For big tracks 1/8th scale, etc. plan on renting a bobcat and a walk behind vibrating roller (unless your 18 and have ALLOT of friends). Tracks take a tremendous amount of work. We moved over 120 cubic yards of dirt.. that's 13 completely full dump trucks worth of dirt!!
* carefull when digging down to deep in wet climates..you will build rc boat tracks.
* try to slope the track in wet climates..have a plan on how to sweep the water off.
* have fun and buy lots of advil before you start anything. My back still hurts!! LOL