So he can say "i built this MYSELF"
Sure... if the parts work well together. If the parts dont actually work well together, the sub could fail or sound horrible. Sure, he could still say "I built this MYSELF." However, I wouldnt be too proud of building a POS.
Op, if you want to build a custom sub, please do some research. Good luck
first one i did i took a WMD2 motor ordered someparts from sundown.. and put it all togeather.. took 7k pretty good and sounded great... cant learn without trying