Whoever said bee is only worth buying with cursed hidan is full of it. Bee is a solid ninja all around who will just get better over time. That said, I wouldn't run him as part of a swordsman team. Lightning is really big on team synergy and each ninja buffing one another. Bee only buffs jinchirki and his own damage is nothing to write home about even if he is buffed himself from lightning ninjas - I wouldn't recommend that team of sage naruto, bee, sasuke and lightning main. Running Iruka instead of bee in that situation is better, since he would be buffing sasuke, and your main with naruto while giving you some much needed first round control (vital with how hot GNW tenten is currently). Sage naruto fits better in that setup since the chakra gain passive allows for both iruka's interrupt and sasuke's field on turn 1 - allowing you to slot out your lightning main's acupuncture for something else. The team is still going to be a bit wonky and lopsided in how it performs but should yield better results.

Wind, bee, gaara, and sage naruto are a solid team that would last quite awhile but it normally uses KAGE gaara, not normal gaara. It should still be alright to run the normal one, but the results will likely be more lackluster. I would recommend switching to wind main at 60 though to run this team over the lightning one.

Regardless, you are using a sage naruto at level 51. You are going to likely spank most all of your competition on your server for a long while, so hyper-optimized team comps aren't really necessary.

For summons go for whatever. They are harder to aim for specific ones, just remember you can redeem your summon runes for summon points and then spend those in the shop for 5 fragments of a particular summon - normal going rate is 1800 points for 5 frags. Over time you'll collect plenty of rare ones so it's kinda irrelevant to aim for certain ones at your stage.

Spend on the monthly card, every month. That should be your number 1 priority. Treasure of the sage sometimes has good deals, but it is always a bit random and you might not get anything worthwhile. Lucky board is really far and away the best event to spend on.

Absolutely. Neji adds nothing that you need with that team right now. You will have far better results with sasuke, you just need him to be 3 stars ASAP.