Just don't enter eshop with 4.5, not even with 4.5 emunand because it will certainly ask for update and if you accept it will update the real system.
Some people recommend keep a 3DS for flashcart in general and other one for your retail games but i think you can do a system transfer on emunand without problems, just understand that some eshop games like pokebank doesn't work. Also, you don't need nothing if you just want to play compatible games, all titles (except pkmn, AC: NL and another japanese one) work fine on 2.0b1 without emunand because the same causes the language error (japanese, like you said), nothing harmful except the language itself.
For bricking issues just keep away from 2.0b2, however you need him to updated emunand/eshop (everyone hopes that one use or two don't harm your portable).
Any more questions feel free to ask
Edit: Just forgot about 2.0b1 save issue, sometimes when getting out of game (you have to press home and then close to properly save the game on the sd card) you can get a system freeze and save game corruption so to be sure aways make backups when possible or use firmware 1.2 (some recent games like LOZ:ALBW, Yoshi's new island and others at same time release won't work at all)