if you already own a few buildings then it probably wont have to be too big of an investment.
your going to be looking at (depending on size) 10-20k in inventory. alot of the times reps will give you even bigger discount for a certain number of displays, atleast thats how rockford is/was. maybe 1,500 for a computer or two. a big money maker is the simple cd player instal or remote car starters, so unless you are going to be doing all the work you will need to factor in a couple employees. to really be succesfull and get your name out i would suggest doing a demo vehicle. a shop near me has a 07 escalade on 6's with the entire cargo area glassed out with JL w7's and JL amps..practicle? no but it gets your name out. you wouldnt have to go that extreme but i think you get the idea.