To answer your questions properly it would be helpful to know how you plan on booting into that emuNAND. There are a few ways to get into it, but most people prefer to use either menuhax (Will allow you to essentially boot right into the emuNAND, or into the HBL after boot up) or go though the long process of booting up using Arm9LoaderHax. (Requires you to downgrade down below FW 3.0 and extract your OTP.bin and do some other things)
Another quick note, the emuNAND itself is basically a hidden partition of your SD card (It usually takes around 1-2GB of space) the CFW (Custom FirmWare) is the thing you use to access and utilize the emuNAND. A few out there are ReiNand, AuiReiNand, RXTools, and Cakes FW. The one you choose to go with just depends on what you want it to do. After going with A9LH I have been using AuiReiNand as it's very simple and seems to have a good support around it right now (Plus it was one of the first to become more stable with A9LH) I used to use RXTools, and it did everything I needed it too, but It supposedly has some more features, that I never really used as I found there were other apps available to me that worked better at the time.
With MenuHax booting takes a bit more time than going right into the sysNAND, it's pretty stable as it's been the main method for quite a while, and the emuNAND you use is really stable depending on the CFW you choose to go with. As far as bricking goes, as long as you still have access to your sysNAND, proper backups, and an entry point then you can go in and restore your emuNAND to a working backup if it bricks for some reason, so it's pretty good.
With A9LH booting times are a little bit faster, closer to sysNAND, and it gives you the ability to open a program like decrypt9 before anything else loads so you can restore your emuNAND or sysNAND backups and have a much better chance to resolve any potential bricks you could be facing.
For your last question, pretty much everything on your sysNAND can be cloned over to your emuNAND the first time you set it up, then from there you can put whatever you want on it. The rest of the stuff you load into it will be stored onto the sd card portion that isn't hidden.
Hopefully that helps a little bit.