I would stay away from a pit of that size with a firebox made of 1/8" steel. Warping is way too much of a factor if it's not properly braced and gusseted. If it's a professional builder who will stand behind it then if the price is right it could be a way to go but if its a backyard build, I'd keep looking. If it's new then it's untested and I know from the first few pits I built, you don't start to see the things that happen to a pit until it's been heated. Look for strong firebox doors with plenty of bracing. That is where I had most of my problems once the pit gets hot. As far as insulation, it's not hard to do yourself. I added firebricks to the inside of my firebox which made a HUGE difference in the amount of fuel needed as well as how unbearable it is to stand next to when it's 90F+ out.
Check gaps on the main pit door for fit and finish. Those round tank doors are hard to tweak if they don't fit right. (once the door is cut, the door has a tendency to spring out making it not fit the old radius.)
Just a couple things to think about. If you are a fabricator or have access to some tools and don't mind tinkering, go for the cheap ones... Otherwise, treat it like a used car and look at EVERYTHING. repairs and tweaks at a metal shop will get expensive!