T304 ss is the lower grade but adequate for headers, IMHO.
Key specs to focus on with headers are:
1. Material specification, flange. The thicker the better.
2. Material specification, tubes. The lower the gauge number the better.
3. Are they coated? Coatings are preferred, such as a ceramic coating, to protect the material from rust, corrosion and discoloring.
4. Do they come with gaskets? Most don't and that's no big deal as the gasket sets usually supplied suck. Get a good set.
5. Collector cone material specification. Will probably be the same as the tubes.
One other suggestion. Get a set of Stage 8 fasteners. These will not back out and create a leak problem.