Does this tank come with a fuel line, filler neck, or sending unit?

Tank only — no fuel line, cap, sending unit, or filler hardware included. You'll source those based on your fuel system layout.

Will the polished finish stay looking good or does it need sealing?

Polished aluminum resists oxidation naturally, but over time it can dull. A clear coat or aluminum polish applied periodically will keep it looking fresh. No sealant is required for function.

What kind of mounting setup do I need for this tank?

Mounting method isn't specified — you'll need to fabricate or source brackets based on your engine bay layout. Bottom-feed outlet position will determine how you orient the tank in your installation.