Personally I wouldn't worry about using the creosoted wood, as by the time you have removed the compost and spread it around the garden, any residues will be so tiny as to make almost no impact.
All compost boxes made of wood rot sooner or later, in my experience, but the only way to avoid this happening too quickly would be to use a hardwood like oak or some exotic hardwood. Really not worth the expense!
One way to protect the wood is to line the compost box with black plastic. You could try putting the posts into those metal supports used for fence posts.. relatively expensive to start with but your posts will then last years longer.
Don't forget that the MINIMUM size for an effective compost box that will heat up is 4ft by 4 ft by 4ft.
The ideal system is to have three boxes together, sharing a common back and front wall, with 4 ft removeable partitions or boards. That way you can fairly easily turn the compost into the middle one and back to the outside one to ensure good mixing and heating up, which makes all the difference in the quality of the compost produced.
All this is a lot of work, but once you have some good compost bins they will last a long time and you will get a really useful and useable product as a result.