Can I use brass valve on my kettle or should I use stainless steel valve?

Answer 1

That depends on the material your kettle is made of.  If it's stainless steel (ss) you can use either brass or ss.  If you have an aluminum pot you don't want to attach either due to corrosion problems. Brass and SS are electrogalvanically similar materials, therefore corrosion between the two is not a concern, but aluminium and SS (and brass, too) are fairly dissimilar and shouldn't be in contact with one another.

Also, keep in mind most brass valves contain some amount of lead (for machineablity) and if you search you'll find methods (hydrogen pyroxide & vinegar) to help remove the lead or just use SS and avoid it all together.

Btw, brass valves are much cheaper than SS and make them far more attractive.