Charlotte
I have a number of raised beds that I made and use happily out of pressure treated wood. There are lots of theories about leaching and so on but the evidence I have seen suggests that if the wood is dried properly after treatment there is minimal leaching. Others will disagree
Unless you are going for some spectacular gardening I wouldn't bother with dovetails either, that seems like a huge amount of work for something that is basically holding dirt...
All I did was get some 12" x 1" planks, some 2"x2" for posts. Cut some 30" posts, cut a crude point, marked out the plot, banged in the posts at the corners and the middle of the longer lengths (8') and screwed the boards to them with exterior screws. Job done.
For the compost bins I have used some pallets, three held upright screwed to stakes with a loose one leant against for a fourth wall, very easy. I would be more hesitant using pressure treated wood for compost bins as you need to turn it more, it can heat up and get traces of organic solvents all of which are more likely to leach out the treatment chemicals. If I was doing it again I would make them a little more carefully, taking the wood off the pallets or even buying fresh wood.
Hope this helps, if not scroll on :lol:
Good luck with the project whatever you do. And get them done quick so the frost can have a good go at the soil before spring!
Toby