I would ask (and I'm not discouraging you by any means) why you are looking only for a compensating horn? I personally chose one because I play a lot of solo material that dips down into the basal octave consistently, and because I'll be keeping the horn for a long, long, long time. If you are ABSOLUTELY 1000% positive you're serious about music for a long time, sure, go after a comp. horn. For most high school level music though, I think you can do just fine with a 4 valve non-comp. You can still play the first octave and you'll build your skills in listening and using your embouchere to tune.
I played a 3-valve Bach in high school and found it to be a nice horn. I rarely need the 4th valve and could have made do just fine with a non-comp. 4 on the occasion the music went down below an E. But if you are in an uber-competitive music program, and attending exclusive music camps in the summer, and gunning for Indiana U or Eastman, then maybe a comp. is a good decision.
Seriously, I'm not discouraging you, just wanted you to think through it if you haven't already. You could save yourself a lot of cash (> $1,000) if you don't need a compensator. I have known too many sectionmates that went out and bought real expensive horns only to not need all the bells and whistles or change their minds about music.
A Yamaha 321 is a good horn you can get at a reasonable cost. I would also recommend a Weril H980, which can be had for an even better price and is comparable to a Yamaha. A couple of compensating euphoniums I would tell you to take a look at would be a used Yamaha 641 (cheaper than a 642 but basically the same horn, now out of production) or a Miraphone 1258. I'm not a Besson fan, but that's just me. Miraphones and Bessons - make sure you play it before buying one. They do have intonation problems occasionally.
Go for it if it the right decision for you, just make sure this IS the right decision for you. Good luck however you proceed.
Looking For Euphoniums
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This forum is for buying or selling your personal equipment. Sponsored selling is allowed as well. All ads are required to have the following information: Price (even for trades), brand, model, and location (City and State, for instruments, not accessories). It is acceptable to link to an external ad if you are promoting a sale of your personal equipment. No Ebay auctions, but "Buy It Now" listings are fine. Photos are HIGHLY suggested as well, and may be hosted on Google Drive, or elsewhere. If you see an ad that does not meet these criteria, please report it.