So I've been interested in mobile security for quite some time now but have only focused on iOS. I've been wanting to get into android so I bought a at&t galaxy tab 4 (big mistake) only to find out its pretty much the worst device for what I'd like to do. I've been looking around on ebay for a "new" android device to buy and I'm not really sure which way to go. I like the Nexus line but most of them are too expensive for my budget at the moment. I did run across some decent price Nexus 5's as well as some LG G3/G4 that I thought about buying.
So my question is what $100 or less device would be the best to buy for my needs (root, custom recovery, ROMs, App pentesting, kernel vuln research, etc.) Any advice is greatly appreciated.
Edit: Please provide model numbers if possible as I've noticed the exact model and even carrier make all the difference when it comes to Android.