Going on experience, check the wood is sound especially around the bottom of the sides and top of the ramp. The newer old type have the metal floor which is less likely to be rotten. We purchased a 510 recently from a friend, we had borrowed it and our horses foot went straight through the floor at the top of the ramp, on further inspection the wet had got in and the wood had turned to mush. We since bought it off them and have replaced the ramp and both sides need doing soon aswell.
On the plus side they are great trailers and tow lovely.
Happy hunting!