How does your computer stack up compared to the minimum requirements on the TF2 store page (at the bottom)?
I think either your short of RAM or your video card RAM isn't large enough so it can't load all of the textures (metal, brick, etc). I can't determine if you need a newer video card, but anything recent (past 3 years) should be able to handle it(if thats the issue at all).
If you meet the hardware requrements, then maybe TF2 got corrupted somehow. Try validating the steam cache (Right-click TF2, pick "Properties" from the popup, find the "Local Files" tab) and then run TF2 again.
Good luck fellow pyro. If you need more help or got it working find me on steam and lets melt the enemy together.