Regarding suspention trailers, I have some ideas that are easy and will proberly work, one is leaf springs, lots of car trailers have them, they come in many sizes, get some old ones remove the smalles piece and use those, trucks have em to

http://www.dodge.com/shared/2008/ram...eaf_spring.jpg

the other alternitive is the springs from a bike I own, they are adjustable to, I don't have a picture on how to install them but put the rear wheels on two long arms on each side of the inner of the wheels and make them pivet from front of trailer with the shocks holding them to trailer by the axel, wich will proberly work good, i have a pic of those shocks but any may work, I have the same bike as this, it has good shocks