Subwoofer suggestion

So I’m basically back and forth between subs. I’m looking at either a sundown nightshade NS 18 in my SUV or I want 2 sundown M12s. I’ve been planning on getting a new alternator for my car as well as I’ll need it for this setup. I was also curious whether yall think I should build my own box, or have a shop build it for me as I’ve never built my own box before. I was also thinking between a Taramps smart 5, or should I go with the higher quality and get a SALT 4 for the setup? I’ll take suggestions for any other subs as well. Budget is around $3000 willing to go a little higher if yall think it’s worth.