First time buying parts ( derailleur, chain, rear axel, shifters) need help! I have a 21 speed (3 front, 7 rear) NEXT bicycle ( model number 8568-49, year 2007.12.06) sitting in my back yard at the moment a few missing parts and I was hoping to get some help on what to get and whats compatible with what because my knowledge of bicycles is limited. Ok so here it is...

Bent rear axle 3/8 thread 165mm... I found it hard to find a 165mm would a 170 or 175mm be alright?

Missing rear derailleur... I no model number to go off of here so I was thinking a Shimano Tourney SIS TY22? The cassette is a 7 speed Shimano MF-T207 and the front derailleur is a Shimano FD TY18.

The twist shifters are bad I noticed they were SRAM so here I though... SRAM MRX Twist Shifters? Will they work with what I have and what I'm proposing in terms of derailleurs?

And finally chain, here I have no clue... something that wont snap as soon as I start to peddle.

I'm looking to complete my bike and hope to enjoy in soon, I'm very open to suggesting and thank you for any help.
There a good chance that the combined cost of the parts you need will exceed the cost of the bike. The Next brand is a brand sold at discount retail stores. They retail for around $125. Many of the parts are not available as new replacements. The easy method is to say throw it away. There are some alternatives though. Many cities have programs called recycleabicycle.They teach youths how to repair  bikes in an "earn a bike" environment. If you don't qualify for the program they make accept the bike. The other option is to contact your local bike shop and see if they have some used parts available. If you check your local Craigs list or similar listing you may find another bike that is free or inexpensive enough that you can put the one bike together with the parts from both.