Membership standards will vary by group, so it depends on which group you mean to join. It is best to contact the group you are interested in, to get specific answers to your questions.
Generally speaking, the 501st is for the "bad guys," and you will find a lot of Boba Fett and Jango Fett cosplayers in their Bounty Hunter detachment.
Rebel Legion is for the "good guys," and there you will find people who cosplay as Mandalorians such as Duchess Satine and Sabine Wren.
Both the 501s Legion and the Rebel Legion require costumes to be (more or less) accurate reproductions of characters and costumes that exist in official films, TV shows, and publications. If you want to join either of these groups as a Mandalorian, you must reproduce an existing character's costume.
Mandalorian Mercs is for those who cosplay as Mandos, with an emphasis on people who invent their own characters.
If you want to create a Mando costume inspired by the Star Wars universe, this is the group for you.
None of the groups require a "professionally made" costume, and none of these groups accepts applicants based solely on their costume. It all comes down to how you look in the costume. If you have a great, professionally-made costume, but it fits on you like a 9-year-old wearing his daddy's shirt, then prepare for rejection. If it looks good on you, then it doesn't matter if it's made of toilet paper. (But it better not look like toilet paper. It better look like real combat armor from outer space.)
Generally speaking, if you submit a membership application and you are not accepted, they won't just mail back your application with a big red "REJECTED" stamped on it, or laugh you out of the room. Most of the members of costuming groups like these encourage new members, especially those with sincere interest. More likely, they might say something along the lines of "
we can't accept you, the gaps between your armor plates are too wide. But here are some techniques you can try, to fix that problem. Or here is a link to a forum where people discuss how to deal with that issue." Just take the advice, put in a little elbow grease, and you'll be a full-fledged member in no time!
Also, don't expect people to kiss your butt about how great your costume is, just because all of your buddies and family members who have never built a set of armor or weathered a prop before, tell you how amazing your costume is. You're talking to people who have seen, inspected, and judged hundreds of costumes. They have higher standards than that dude who thinks a shadow trooper is Darth Vader. Don't take a rejection personally. They actually want you to join their group, they just want your costume to be a little better first.
Here is some info on what the Mando Mercs are looking for in a costume:
http://mandalorianmercs.org/get-involved/costume-requirements/