Neither was I, I used to struggle to do forward rolls before Judo and now I can do backward rolls and jump off my arms into standing among other things. Judo is perfect for what you want but you'll have to be careful for injuries that might set back your law enforcement career. Gradings differ between countries and governing bodies. In the UK it used to be if you beat a belt at a grading in competition and passed the theory then you got that belt. Now, you have to pass the theory (be able to perform a few set throws when asked and be able to translate some Japanese words). I believe you also have to enter at least one competition at your level for each belt once you get to green belt and your instructor isn't supposed to grade you until you're fighting at the required level. In the US, you can grade on knowledge or competitively. I'm not entirely sure how they do it but I know that if you grade competitively it's a lot quicker. As for getting the basics down, it really depends. A first dan in Judo is someone who has mastered the basics. But if you're meaning at what sort of level do you have a basic understanding of throws and be able to perform one or two in randori I'd say about green belt level. One thing I've just learned recently is that uchikomi bands (you can use therabands) really help you improve minor issues and overall improve your turn ins. I wish I'd known about them when I was just starting Judo. The only thing you need to be careful of is that you're not repeating bad habits so is suggest doing Judo for a bit and when you really nail a turn in ask your instructor to watch you and then you can start going home and practising it. Apologies for any typos, posting from my phone and won't be able to edit on dubs.