I'm still concerned about what was in it before and why that occupant vacated.. if you know what I mean. If it were me I'd try to sell it cheap and get some of my cash back.
If you really want to try to clean it and use it, you need to F10 or Chlorexidine the entire thing. I'd strip that silicon out and redo the seals, as stuff can over time get underneath it and just live in there (this is why I don't buy used fish tanks typically).
If you really want to use this thing I wouldn't take any chances. Sand the entire thing, and either use a acrylic or marine grade paint (anything with low or no VOC) OR (what I would prefer if I were you)stain the wood then apply a wood sealant. You can find wood sealant at most hardware stores that are "baby safe" once it dries.
Edit: Just to add to be SUPER clear, cure time for MOST acrylic paints is 2-3 weeks. On top of that, oil based stains are toxic, and even some water based can be if you don't properly seal it, so you're taking a lot of risk, OR waiting just as long to DIY this enclosure as you would just ordering something new, that you don't have to worry about the safety of your snake in.