I would make friends at the local performance shop and then save up for a low compression forged 4.6 motor. I say 4.6 because you can use the stock heads, maybe throw in a supercharger grind cam and then save up for a supercharger kit
If I had to do it over, I would have done that. I went with a blower first, started at 8 psi, then 10, then 12 and over boosted blowing my motor up. Power is never enough, especially when you first experience it. Kinda like getting laid lol
If you have the patience to stick to a long term build, I would go that route (forged), then blown. Power wise, I think high boost can make 600-650+ whp depending on the blower and tune on a forged motor.
The new 5.0 is amazing but imagine blowing it up and trying to replace it. I replaced a 08 4.6 with an 07 for $1,899 off Ebay. Did the work myself at a shop I was working at, me and another mechanic did. No way I'd be able to afford a new 5.0 or attempt replacing one with my own cash. Just sayin'.
I ran about 500whp on my 08 before I sold it. It was quick. Quickest car I owned, but it wouldn't hold a candle to a boosted 5.0 nowadays. Crazy how cars become so advanced, it's hard to keep up. If you like your brothers 5.0, a boosted 3v will be just fine for you but you'll get used to the power very quickly and want more.
If you can afford the new 2015 I'd go for it as well, only drawback is there's no telling what the future brings and it may come with a host of it's own problems being a new model year. 3v's, things are kind of set in stone and things are figured out. The 4.6 been around forever. The new 5.0 would be great if you kept it basic bolt on's but for more serious builds, it may require more precision and skill, especially helpful if you know some tuners.