I can see where the car springs would work but I think shocks will be a must with them. Longer springs, more flex. Where they will definitely smooth out the ride they will also be very bouncy. Not the trailer spring bounce but more of a long smooth bounce if you get what I am trying to say, so you'll want shocks to stop that. Take the shocks off the back of a car and drive it once, you'll understand (been there, done that, weird ride).You are probably right about them being better for offroad as they'll be a lot easier on the trailer (smoother). You will most likely have to make your own U-bolt plates because trailer springs are narrower than most car springs. A good choice might be from an 80's Caravan.
As far as ordering an axle, your best bet is to surf around and check prices AND shipping cost and go with whichever total cost is the best.
The strongest reason for the people to retain the right to
keep and bear arms is, as a last resort, to protect themselves
against tyranny in government.
- Thomas Jefferson -
Personally, I carry a gun because I'm too young to die and too old to take a butt kickin'.