Dumbells is the first thing I check. If the weights don't go to at least 130 I'm walking out of there no matter what.
Squat racks for me is next. If they dont have at least 2 or more squat racks, i'm not joining.
I personally find the newer plates with handles in them, or holes in them an attractive feature for a gym because I can do all kinds of odds and ends holding them.
Deadlift platform is also a very nice necessity if you are going to have true lifters there.
Chains and Bands would bring in some respectable lifters also.
A few of my strongman friends when they built there gyms, I was there and watched them outfit them. Poundstone from WSM and Mastell a LW pro used SORINEX equipment to outfit there gyms. Very practical and less expensive but super well made. Mills another LW pro in my area used alot of "ROGUE" equipment. They have beautiful and amazing equipment but Expensive. I also like Watson equpiment and Dumbells they are based out of the UK but they have AMAZING equipment. Most beautiful dumbells I have ever seen are made by them. So those are some options for serious equipment.
Here's a link to Derek's site where he tells what equipment he has, this might help give you some ideas:
Equipment - Poundstone Performance
(the dumbells below are WATSON brand, they say 'evolve' but that is simply the name of the gym and endcaps that Watson made these for)