I have played (do play) tuba (primarily), bass trombone (secondarily), and euphonium (???). I'm currently devoting all my attention towards getting "reasonably good" on euph before my shelf life expires.
Brand? Depends on what you decide to play, how much money you have, and what's available. See my signature for what I've been playing over the the past ten years.
This question doesn't make a lot of sense. It depends on you and the horn. For my current (primary) Wessex (Besson 981 clone) compensating Eb, I use Wick because that's what's best for me and that horn -- basically it was made for the horn. I usually have the 3XL in it because over the entire range (FOR ME) that is best. But for contrabass register intensive pieces I may stick the 2XL in it since that works better under those circumstances (FOR ME).
For my 1924 Buescher Eb 3-valve, I have found that a Kelly 25 or Bach 25 works best. I like the Kelly because I like plastic rims. This mouthpiece would be dreadful (at least FOR ME) on the big compensating Eb or for the BBb horn I used to play.
On full-size BBb horns I have used a variety of mouthpieces, including a couple of Perantuccis and Schilkes. I CANNOT use the extremely popular Conn 120S -- it seems to simply suck the air out of me. My absolutely all-time go-to mouthpiece that I know will work to some degree on any tuba for me is the Schilke 66. It's a "smaller" Helleburg. That doesn't mean that any of these will work for you. It will take time to figure out what will. Start with a Bach/Kelly 25, learn to play, and then figure out where to go from there. The 25 won't overwhelm you in size, and it will be reasonable. DO NOT go in the direction of a 24AW.
I cannot see myself ever using any Wick on an American or German style BBb tuba.
Wessex, Mack Brass, ... ??? Or even Jim Laabs (Schiller) if you feel adverturesome. Or find something used that will work for you.
From the vibes I'm getting from your query and remarks ... if I were you, I'd look for a smallish 3-valve BBb horn, stick a Kelly/Bach 25 in it, and start playing. An old Yamaha, Olds, Eastman, King, Dillon. Take a look at Dillon used tubas. I have no idea of how much you're interested in spending or how you'd come up with the money, but in your position, I'd be cautious about spending much on a "first tuba". Just my 2 cents.
