Marelon valves on plastic through-hulls is a good solution. I had these in a steel
cruising yacht for >13 years and had no problems at all. If they are to be fitted in the engine
room, consider the consequences of fire. If you do opt for welded s/s nipples, ensure that you use the correct electrode. 308s or 309s i believe are still the recommended rod for welding s/s to carbon steel
. But check with a good welding consumables supplier.
I dont see enough advantage if any for using s/s.