Let's start with horns: I recommend the classic Besson 955 Sovereign (3 valve compensator) or its equivalent York 3055. Personally, I play a York 3056 (4 valve same as the Brsson 956). I generally don't recommend the older 4 valve compensating baritones as they are pretty awkward to hold and the 4th valve isn't that terrific as the compensating loops are not appropriately sized. The new Besson Prestige 2056 is a superb horn, but is quite a rats nest of tubing. Taking off the bottom caps on valves 1-2-3 is a real difficult task! Also, the Pestige is quite pricey. The Yamaha YBH 301 plays pretty well but has a pretty brassy sound. On a good Besson/York, I can definitely get a nicer, rounder sound and feel I can play across a wider dynamic range. A lot depends on what your seat mates are playing. If they are already Yamaha, then the 301 become sa very cost effective choice (used at around $1000).
As for mouthpieces, I am doing solo work at NABBA and with the band as well sectional work on 1st baritone (while still playing euphonium). I use either a 4AL or Alliance E3 on euphonium. On baritone, I found the Wick 4BS and 5BS too large for me. The Alliance B6 or Wick SM6B work pretty well. But just now I am playing a Bach 5G. I find it easier to play very soft (necessary for inner harmony parts) and I can get a very nice, centered, warm, pillowy sound across the range. I have experimented at length over 4+ years with Wick 4AY, 4BS, 5BS, SM6B, Alliance B6, Schilke 51D, Bowman BB1, and Bach 6 1/2AL and 5G. Of all, I think the 5G works best for me.
Doug
Adams E3 0.60 Sterling bell - Prototype top sprung valves
Concord Band
Winchendon Winds
Townsend Military Band