Having purchased those barrels from Sun West myself, they do not have a liner of any kind. Bare metal on the inside. Not even an oil coating. Your seasoning method is sufficient. I found a light coating of cooking spray helped the process prior to seasoning. Make it a long empty cook if possible. Another option you have is whether or not you want a bung hole (insert joke here). I have both. My bung hole lid exhaust helps with higher temp cooks, 300 degrees plus, while the non bung hole lid has drilled holes, better for lower temp cooks. Either one I can control at any temp.
If you need any other advice, don't hesitate to ask. There are several of us drum heads here who have tips to help you avoid the pitfalls.