My Ds is almost 18mths and we have just bought a bed and trundler set from fantastic furniture. It didnt include mattresses but it was $269 for the set. We have a decent mattress that we can use and then will get another one for the trundler.
He will be usung the trundler bed at first(its really low to the ground so he can get on and off himself and for the falling factor as well) to get him used to the big boys bed then when he is ready or wants to he can move to the bigger one and we have a spare bed.