starter relay needed? ????

I lost you on how the NSS fits into your plans. Don't run anything but a small relay coil current thru that.

You could probably control the starter solenoid directly w/ a 30A manual switch. I might guess it draws ~20 A. The main starter current (>100 A) will still flow thru the thick cable from the battery I assume. You must have a relay to use the NSS, and probably not safe (or allowed) to not use it. You can use a standard 40A "automotive relay". They do so in modern Mopar cars.