Hello Tom,glad you dropped in. The short answer is redwood and no finish,varnish or otherwise. The problem with finish isn't just about toxicity. Plant containers must be porous to air and drain well. I highly reccomend you visit "container gardening" where you will information relivant to your project. Any number of woods are suitable,each with advantages and disadvantages but redwood is the hands down favorite. Osage orange and Cypress are last long and look rustic but difficult to find. Pine and several common and inexpensive will work albit short lived. There is one that comes to mind you should not use,CCA treated or any rot and insect resistive treated materail. The project as a whole is a little more protracted than what wood to use.