I didn't read all the other posts, but here's what I've done.
You could hire a sound guy who brings the PA and runs it. You have to either deduct it from your pay or charge the venue the extra money for it. Pros: You don't have to buy it, lug it and fight over how much to sell it when the band is done. Cons: Long term, it could cost you more money and some gigs ie: You can't get a sound guy for a particular gig, the venue doesn't want to pay the extra money for the sound.
Buy it as a group with equal shares and sign a contract. If one member leaves, the band pays him the current market value for his share (depreciation built in). The new guy could buy his share and enter the contract or the rest of the members now own it. If the band breaks up, you sell it and pay everyone equally. Pros: It's equal Cons: Complicated
Buy it yourself and work out how much extra you get per gig for providing it. You may have to set it up and break it down, but you can possibly get an extra cut for each gig.
Pros: You make extra money Cons: Band feels cheated that you get extra money, and you have to carry the stuff.
Good luck.