The endorfinders gloves are great, and you could abosolutely be approved with those (no item is "501st approved" btw, it has to fit you). Boots can be found at imperialboots.com.
There are several people who make helmets, like JJ Industries on fb (Luke Daley), who I think has the best one currently. Kylo's helmet is pretty small, so size is not going to be a problem if you have a small head (you need to use padding regardless). You could add a voice changer to the helmet seperately (though your phone, for instance, with a mic in your helmet and a small speaker hidden somewhere).
Also, you should join the Knights of Ren fb-group, and also go to http://forum.501stsithlords.com/
(super important that you read the kylo-sections there.)
The main thing when building a costume for the 501st is to do a lot of research before buying or making anything. You should know the costume inside and out, so that you get it right the first time and don't have to waste time and money on mistakes (best advise you've heard today).
Good luck :-)