Reddit mentions of 5.11 Tactical Rush 72 Back Pack

Necessary? No. You can do 'MMA' with no protective gear in the buff. I wouldn't, but you can.

Useful? Sure.

Here's my take on gear:

Grappling gloves: Definitely should have a pair.
Head Guard: Unless you like getting concussions, this is useful.
Ear Guards: I don't own this currently, but it's a useful thing to have if you want to avoid looking like a fighter (cauliflower ear can affect employment similarly to facial tattoos in some situations). You can get cheap wrestling headgear though, no need for special mma earguards.
MMA Shorts: grappling shorts? yeah, you should have some shorts. Do they need to cost $50+ a pair? No. Cheap $20 shorts are fine.
Kneepads:Not necessary, but if your knees are getting busted up, wear some. Again, you can get cheap ones, so it's not a big deal.
Mouthguards: A necessary item. You don't need to spend $100 on a custom fitted one (though people claim those are very comfortable), but you do need to have one. Use it all the time. It's the little accidents while rolling, or the accidental hit when drilling that can cut off your tongue or knock out teeth-not just the big fights. A cheap boil and bite is fine when you're beginning.
Shinguard: Necessary. I don't care if you plan on kicking trees until your shin becomes made of adamantium. This allows you and your partner to spar a few rounds and still be able to walk tomorrow.

Other necessary items:
Compression shorts: Unless you're going with vale tudo style shorts, have something that won't get pulled down even if your shorts do.
Cup: A lot of guys like the muay thai steel ones, I like my nutty buddy, find one that works for you. Some people say to not wear one. If you're done having kids, maybe. Really it won't be useful most of the time. That one time every couple months that it will catch something is well worth wearing it every day.
Sparring gloves: 16oz usually, depending on the school.
Rashguard: I don't care if you're doing gi or no gi, you should have this, IMO. A cheap walmart one works fine though, no need to spend for an MMA one. Just make sure it's heat dispersing not heat retaining.

Personally, I also own some thai pads, focus mits, kick shield, tombstone pad, and I'm planning on buying some mats in the next month or so.

That said, talk to your trainers. Don't buy any gear immediately. Any gym will have loaners of all the basics. Go in there, get your ass kicked for a while, and then figure out what you want to do. A gym may specifically want you to have this or that that's slightly different from the norm.

Edit:

A good bag makes a world of difference. I have a great duffel that has a separate compartment for clothes so they don't get mixed in with my sweaty gear. I love that. It also fits shinguards/headgear/mma gloves/16oz gloves/etc with no problem. http://www.amazon.com/Hatch-Mission-Specific-Size-Black/dp/B000MAXN2U/

Some people prefer backpacks, and my wife uses the Rush 72 for BJJ, I think it could hold most of my gear also. Great bag for traveling as well. http://www.amazon.com/5-11-Rush-72-Back-Pack/dp/B005AG3YQG/

Why not take a 127 of your 500 dollars and buy yourself a 5.11 72 hour bag. if that is a too much then I suggest hitting the local military surplus store and buying/building an alice "hellcat"(google it).
You seem to have everything but I get the feeling your lacking in having tried these items out. May I make a suggestion. On your off day, say it's saturday from sun up to sun down live out of your bag. And I'll tell you the things your going to see right away.
Your lacking a more solid and durable utensil and mess kit. Find something metal for your fork or spoon and add a squishy bowl or folding platic plate too.
You need a second mode of holding water, add a water bladder to your bag (wait till you get a new bag before you buy it) your itemization is a bit off. I suggest making a small holding bag for every catagory. Your lacking in the ability to carry anything extra with your bag. I'd look up furoshiki to help you with that. Your food choice is really bad, the weight of your soup is a bad choice your bars are not a meal replacement and they'll hurt your moral over time. Think dry, think moral when it comes to your food. Here's my plan for you, take 250 dollars and buy a good rush 72 bag and a food thermos the biggest most solid one you can find then add a nice solid kabar while your at it. If you shop around all three will set you back about 250 bucks.
From there go out to your local grocery store and pick up some lipton chicken noodle soup, the dry packs that just need hot water. Pick up some uncle bens boil in bags rice. Then add in some grains that are easy to cook. Switch up your bars for those beefy weight lifter kinds.
If your not going the mountain house or MRE route then at least get the best over the counter food you can.
After you look into furoshiki buy a measure of cloth and practice wrapping shit.
While your at the grocery store buy some thick straws, cotton balls, petroleum jelly, and a square altoids tin. Then look up how to make some fire straws.
You're better prepared then 80% of the population but lets boost you to 90%.
If your really really cheap start at the 99¢ store and thrift stores to get all the things I suggested. But the three things you should spend money on are your knife your bag and your cooking set.

I too have been on a bag hunt...



Knomo Jackson Briefcase

Far Horizon Traders Journeyer

Filson Padded Computer Bag

Osprey Quantum Daypack

Briggs & Riley Live Large Backpack

Bushcrafter Pack

5.11 Rush 72 Backpack

Red Oxx Sky Train


Mission Workshop Vandal

And finally the Goruck GR1


I ultimately decided to go with the GR1 for a variety of reasons

5.11 Rush Backpack. It has a bit of a tactical feel to it but if that's your taste, it's awesome. I have the Rush 12 but can imagine that a 24 would be more suitable for your usage.

• Rush 12, Small
  
• Rush 24, Medium
  
• Rush 72, Large