Hardwoods like Oak are the best, Birch is fine, but it depends what you are looking for... I made one to match a rustic mexican pine living room set and I just used the best pine boards I could find at the Home Depot, knots and all. DO NOT get concerned about weight, you can't build it too heavy. The glass of any tank is super strong on the vertical plane. My LFS guy actually stands on the bottom row of ten gallon tanks in his store to reach the upper tanks, I see him do it every time I am there. There have also been people who put two 10 gal tanks directly on top of a 55 gal tank without a problem. My own canopy weighs over 30 lbs after wood/lighting/fans, and it has been on my 55 gal. for three years w/ no problems.
As far as treating it, I hit the outside w/ oil based stain and seal it with polyurethane in either gloss or satin, two light coats is enough (it's not a floor you're sealing here, it doesn't have to be built up too thick, just close the pores on the wood... You can touch it up a few years down the line by sanding it and applying additional coats). I spray paint the insides with gloss white Krylon, a couple/three coats does a nice job of sealing moisture and reflecting light.
Regarding MDF/Particle board, my buddies' 180 gal has a canopy AND stand made from MDF, painted with Fleck-stone spray paint... It looks nice, but you have to route the edges to a radius for it to look good and help shed water. The stand is entirely made from MDF, basically a box w/ no structural support/framework of lumber inside it at all... It's been holding his tank in the same spot in his living room for at least 10 years now.
Good luck & have fun it's a rewarding project.
