Good guilds that want to actually fight for high rank/position, probably wont accept new players. Some of them even have discords channels and require you to for example do 2 or more fights a day, or reach certain contribution. Those really hardcore probably wont even accept good veteran players if they don't poke their face on discord or something.
Personally I only applied to one guild, and after 3 days of not getting any response, I just applied to guild that had more or less good info/rank, had lots of people in it, and had auto accept on invites. Got in and learned fast that guild is basically dead, half the people on edge of being kicked out of inactivity, and most people showing up online like once a day just to get daily login bonuses I assume. A.k.a. I was the only one doing anything in actual GvG battles. Nowadays few more people joined (probably in place of people who were kicked out including I think guild creator hehehe). And few of them try to help a bit.
I don't know how coins reward work at all. But medals work in a simple matter - after each of 4 round of battle you get medals depending on your place in GvG and also your guild rank. There are 4 tiers in which you can be based on rank, and depending on that and what place you end up you get medals (or not as some tiers give no medals on lower positions). Tier/Place/Round info how much medals you get is in Help (under the game) in section Guild Wars/Military points rankings.
I personally have zero knowledge about power of the girls. All I know is 5* are better than 4*, and all other things are probably leadership/skills depended, so also your team composition is important. Frankly I don't know why the heck there is a 500 medals and 360 medals girls to buy. Maybe to have "more incentive" for people to do GvG in hardcore mode and farm them, cause you need multiple copies of each if you probably want to have them maxed out as you need to have Luk increased to have skills level cap raised to up to 13.