Re: BI amount: I agree in theory with
@Jon-PokerVIP, but I'll offer the counterpoint that the live and online experience is so different that even if you're a crusher at NL50, you'll still be a beginner at live for a while, and there's absolutely no shame in not buying in for the max right away. Buy in 75BB, wait an orbit or two, and see if you're in a good seat to top up or not. If you have an aggro or two on your left and they seem pretty competent, you might not want to top up until you can get a table or seat change. There's one well-known player/commentator in California who always buys in 80 BB until he has a sense of what's what, and he's way better than me, so I always think about his strategy...then buy in for the max 150BB anyway.
(related note is that you'll quickly learn that playing 250+BB deep is a much different feeling, and esp when you're in the BR building stage, there's a tendency to play a shade tighter than normal and protect your stack. Soon you'll be comfy donking off 300BB with the best of 'em. :P)
Re: BRM: 20 BI is is certainly do-able, but near the bottom of where you'd like to be. It's a reasonable starting point for a "decent amateur" player, which I'm sure you are. I'm going to cut and paste something I posted a few months ago from my Live Poker Containment thread, since I was in a spot similar to where you are now.
BRM: I'm not practicing BRM for my live game. I don't want to touch my online roll, but I took $2K in profits out, and decided to use some of it to "take shots" at 1/2 live. (I'd been to the casino before, but again, just taking a shot once every few months.) Now I can mentally consider myself freerolling, thanks to a hot month and V's who love chasing gutshots, while at the same time still acknowledging that I'm still in the taking shots phase, even if it's 3x/week. I can envision a time when I decide to properly set up a live BRM plan, esp if I decide to hop to Vegas for a few days, which I would prob only do if I have enough profits from my current game.
Theoretically, proper BRM for 1/2 live is anywhere from 20-50+ BI, depending on what your goals are, and also how close to bust you can get before you mentally decide you busted. (It does no good to have 20 BI but when you lose 10 you freak and quit the game. Turns out, you honestly only had 10 BI after all.) 20 is low end for a rec player who is decent and TAGgy, which is where I'm at, so I could actually start proper BRM now. But TBH, I think there's a bit less pressure the way I'm mentally doing it, and less pressure lets me stay TAG and not go into nit mode. It's getting easier to just shrug when I lose $100 as pfr + 2 barrels with air, but it still helps that I think of it as fun/learning money and not BR. I would totally be the guy in my example, says he has $3K to lose but quits after $2K. So I just don't go there yet.