Here's a good article on what wood is safe Caudata Culture Articles - Wood in Vivaria ocean driftwood is not recommended, freshwater driftwood is fine.

Don't worry about him being lonely. Apart from yearning for someone to have a one night stand with, they don't really care about other frogs. It's no problem to keep him on his own. But don't let that stop you from getting a bigger tank so you have space for two

Lots and lots of high up perching and hiding spots are in order. I keep one in a viv with fake rock wall made from cement (pics in my vivarium album). My frog spends roughly equal time on the rocks, the plants, and the tree branch, so I don't think I'd say he has a preference apart from hanging out in the upper half of the tank most of the time.

Pothos is the standard vivarium plant. Nice leaves to rest on, and super easy to grow if you are new to keeping plants alive. If you want a light to help your plants out, a simple compact fluorescent works just fine, get one of the cooler colour ones (5500-6500K colour temperature).