This is going to sound like a cop out, but it depends on who inspects your Brewery prior to start-up. We have always used the kegged volume as tax determining because we don't have serving tanks on any of our premises. However, some of my colleagues haven't been so lucky. Even though they keg only, they still have to label their tanks as a "Tax Determination Tank".
Pretty much any stainless vessel with a site glass that you can calibrate will work. I'ld recommend to at least have a vessel designed to hold beer (tri-clamp fitting, et al). Inspectors will generally as how you made the barrellage calibration along the site glass, and you should do this with a bonafide method, such as using a portable water meter. Be accurate, as you are paying tax on this volume and creating suspicion with the tax folks does none of us any good.
If you can go directly to keg as a tax determination, that's the best route. However, you might be required to go to the TD tank prior to kegging. Looking back, I have predominantly seen TD tanks used in places where the beer has a direct supply from some portion of the Brewery or from a walk-in near the Brewery.