Tall J, I hope the time between jobs does the trick for you. Keep in mind 2x4s and 3/4 inch plywood will get heave if you make the modules very large. Lots of folks have used foam like you are saying. I have always used Homasote as a track base , cutting out it and even plywood for below track scenery. The foam will help reduce weight though, for sure.
That is great you have so much from gifts through so many years. Yes, now that you have matured, your interests have developed to a point, some of those items will probably stay in the box and not be used. You have some work to do going through it all. I have found this Forum to be the best place to find buyers who will work with you on price and are honest in their dealings. Who wants to be black balled from the Forum? 
