I bought an older New Braunfels offset from a member here for $150 a while back. It is an older one, 3/8" plate, very heavy. After 2 or 3 cooks I was ready to scrap it, wouldn't hold heat, fuel just smoldered, etc. I did a LOT of reading and found a few mods that were cheap and made it a dream to cook on. I lowered the stack to grate level inside, raised the stack outside, built a "charcoal box" out of expanded metal that would raise the fire 4" off the bottom. I made a convection plate out of some scrap I had and now I wouldn't sell it for $500 because I couldn't replace it. Whichever one you buy just make sure it isn't the stamped steel "Big Box" ones, they just cannot hold heat and will rust through before you ever get it to work.
Tuning plate(s)
Stack before(Rigged to see if it made a difference)
$12 worth of exhaust parts and a little rigging for "roof"
Finished products