I do a little bit of woodworking on the side. I built a box for out of cedar for a small outdoor freshwater container pond. When doing that, I used 80 grit, 120, then 220 for finishing. Did a tung oil finish on the outside. I'd stop at 120 grit if you're going to paint it. The only tools I have are a power drill, a random orbit sander, and a hack saw. And I don't have a garage, but I made it work.
That said, making a piece of furniture to hold a large and very heavy tank is risky if you aren't experienced. Not that you couldn't do it...just make sure you do a lot of research and maybe find an experienced wood worker to talk to.
For my 10 gallon reef tank, I actually purchased a small accent cabinet and reinforced it with braces and small boards. This is because my tank is a really weird size and Lifegard doesn't make any stands specially for them (like why though?), so I had to figure something out. Another note: you can always try looking at thrift stores for old pieces of furniture then customizing them to your needs. They made furniture really sturdy back in the day...