Build the shell of the enclosure as big as you can. Then baffle it and brace it. Load the woofer and run a sweep and see what your peak is... Then tune your wall to your peak... Make sure you seal it off around the enclosure so it's a true wall. Play around with port size. Most spl builds use huge ports.
Big mono amp $2000
Woofers pick up some used zv3 18s for like $700 for both if u can
HO alt $500
Three g31 batts $600-700
Wire and lugs etc $200
Cheap 4 Ch amp $150
Cheaper speakers so u can hear the music $150
Head unit $200
Then all the wood and glue and screws etc... $500+
So u can see it adds up quick!!!! Almost none of us just start out and shoot for 160db out the gate... We started with hand me down boss audio and kicker and sold and traded and worked our way up to big boy systems.