best example of jerry i can think of being both a gentleman type and goofy in the same go is
this
I don't know if I really feel brook from that though.
I'm personally thinking it might go to someone like Robert Bruce Elliott. He has a lot of experience with longer running series, has played old men, as well as men in their 30s-40s, and has played some goofier characters. The only potential problem I could see is he has already done the role of Edward Newgate, but that might be a minor enough role that he could also do brook.
In general I don't figure the singing part is that important. Most voice actors can at least carry a tune even if they can't sing well, and brook to me doesn't sound like someone who sings well just someone who can carry a tune... so yeah...
most of the other main funimation people I can think of are already doing a main role lol (I'm figuring it has to be one of their more popular VA as they have a contest to guess it.... watch... it will be the voice actor of Goku
)