You should check that tank out and if it's not rusted, use it as an air tank or even sell it as an air tank. It was built as an air tank by a certified boiler company. I bet you could get $300 just for the tank if it's in good shape. A new one that size would be well over $600. If you hooked it to your existing air compressor you'd have a lot of reserve for just about anything, like sandblasting. You're not allowing nearly enough for your labor to build a BBQ in your price.
The castors only needed four 1/2" welds to be more than strong enough but another option would have been to just make legs and have a slide out or removable handle and move the BBQ like a wheelbarrow. Maybe you could add something like that? The big wheels are good for moving it outside but the castors will dig in the lawn and/or won't roll on a gravel driveway, especially with it being so heavy. I'm not sure why it needed a heavy base? I imagine the tank is strong enough you could have welded legs directly to it and saved a lot of work. It's good practice for you though and I'm sure the Hobart school will be a big help with your welding skills but I hope you're going to Hobart in order to have a better chance at getting a full time welding job. With the low prices you charge, you'd go broke if you had your own shop. The quality of your work is really good just a little overbuilt sometimes.