I want to make an enclosure for my bearded dragon when he gets older(currently 4-5 inches long) I was thinking out of melamine or plywood, but I want to make it a vivarium with dirt and live plants he can eat. I’m currently at a standstill debating what I should make it out of and how big
Melamine can get pretty heavy so I would use plywood. You can paint it or cover it with polyurethane to protect it from moisture. If you want to set up a bioactive substrate I suggest building one a bit taller (4' or so perhaps) so you have enough space to build proper drainage layers. Live plants can be tricky because they tend to get trampled easily when they are in an enclosure. You can certainly give it a try but it may not work out as you're hoping.
It shouldn't be a big concern at the temperatures you'd normally keep the tank at. Do be sure the polyurethane sets and dries completely before putting the tank into use though.
I'm not going to lie, that is going to be tough. Live plants add humidity for one, and you don't want continuously wet/damp substrate. I would highly caution against it, though it is your call
I’m not sure because I’m also leaning towards buying a pre-built tank but the recommended size tank for a bearded dragon (2x2x4) usually go upwards of $300
You'll be better off building your own in terms of cost effectiveness. That will save on labor and shipping. Taterbug has a 4x2x4' (I believe those are the dimensions) and she has a bioactive substrate going in that. https://sites.google.com/site/thelizardmadness/photogradient
Going back to building my own enclosure, i have limited space to go wide and long, however i do have a lot of space going up. So would it be a bad idea to make a 2 story enclosure with a ramp leading up to the second one
Not necessarily but you won't want to make the ramp too steep. The upper areas should be easily accessible. You could use branches to allow access to higher basking platforms too.