Urea Storage Tank Material

How large of a tank are we talking?
What are the purity requirements?
What will be your temperature control?

Still in the estimate phase of the project, but plan so far is a roughly 12k gallon tank for an industrial grade application. Temp control is dependent on material, if it's a steel tank then we'd have a half pipe jacket to keep it from salting out when the temp outside drops.

Polypropylene is the least problematic and lowest cost tank type. You will need to prevent temperatures from dropping below 40°F. In my region that's not a problem, even without insulation. Urea is non-conductive so you can't use galvanic or impressed current corrosion protection. The tank needs to be built from corrosion resistant materials.

If it were to be made out of steel, would 304L be sufficient then?

I see non-metallic as the best first option.
You may need to have a steel frame fully enclosing it for support.
How clean is your Urea?
How clean do you have to keep it?
What is the external environment?

I recently asked these questions. I have some 304L SS urea tanks that were welded using backup bars in the tank top. They're also skip welded on the baffles. The assurance is that urea has essentially zero chloride content. Urea isn't as corrosive as most people think. Don't use bare mild steel and don't use copper alloys. If you can follow those rules you'll probably be fine. I inherited some coated steel tanks as well. I don't see urea as being any more corrosive than fresh water. Urea is less conducive so you cannot rely on anodes for protection. I've installed magnesium anodes in fresh water tanks as the coating fails. This will not be an option with urea.

The only problem with using 304L tanks for Urea is that if you get any impurities then the welds are likely to be attacked.
In a clean environment 304L will last a long time.
Some coastal plants are using lean duplex (2101) for such tanks as they are worried about chlorides on both sides.