It really depends on how much money you want to spend. Many used boutique basses go over $3k. I'ts possible to get a decent used bass under $2k.
The easiest to find will be Yamaha 620, 613, 611, prices range from <$1k to <$3k. These horns are "inoffensive". I don't think they're great, but they're certainly not bad. The market is flooded with 300 and 400 series Yamaha single valve basses. Yamaha has this tendency to call big bore tenors "Tenorbass". Don't be fooled. These are tenors.
There's also the Bach 50b variants 50b2 and 50b3, along with more exotic valve options. Many people feel these can be inconsistent, so be careful if you buy online. I bought one and loved it. Here's one I'd look at:
https://www.ebay.com/itm/VINTAGE-BACH-S ... SwbJheWbbT" target="_blank
The Conn 62h has a couple versions, the old Elkharts which are venerated and the newer ones, which are readily available. Conn 112h shows up often.
There are two main Getzens, the 1052 and the 1062. I'd recommend the 1052 as it's kind of standard. The 1062 has a giant bore bottom slide, and might be unweildy for a casual bass player.
I play a Kanstul 1662, which is a copy of the old Conn 62h. There are a few around you can find used. Kanstul is out of business.
King has a 7b, which is a pretty standard bass bone, although maybe has a smaller bore through the valves. 8b is a bigger bell and valve bore. The 6b is known as the Duo Gravis, and is much beloved by people who love smaller bore bass trombones. This is highly sought after for lighter, big band stuff. You can generally get them for around $1500+/-
Benge was related to King, and the 290 had a big bell, but is generally highly regarded, although I played one and thought it was unweildy. These are usually reasonably priced ($1k - $1500)
Holton makes/made the 181, 180, and a number of older horns which are again highly regarded. You can get the 181 or 180 for generally less than $2k.
Olds made some double valve basses, but these are more for niche players or modders. I love the p24g, and own a s20. I wouldn't recommend an olds for a casual player. The can be tough ergonomically speaking, and mechanically quirky.
If you want to go Chinese, I'd recommend the Wessex pbf565 is really nice ($1800), and the pbf562 is adequate (~$1k)
I've heard good things about Jupiter 1240
Eastman are generally decent horns.
JPRath/Packer and the lower (x00) Raths are said to be great.
Horns without a name are generally Chinese, and may be ok, or may be trash.
The condition of the slide is important. It can be repaired or improved, but not by every technician, and not in every situation (depends on the type of damage).
And then there's the whole dependent/independent valve arrangement question. Dependent valves require the first valve to be activated before the second valve will have any effect. Independent means the valves can be used independently or together. The biggest difference is that independent gives you a lot more options, which can be good or bad. If you're a casual player, I don't expect dependent or independent will make a lot of difference to you (are you really going to engage just the second valve by itself to get Db and Gb in 1st position?)
The most common tuning is Bb/F/D (the D is for both valves, sometimes independent horns will be listed as Bb/F/Gb/D), although Bb/F/Eb and even Bb/F/E tunings are available. I own a dependent Bb/F/Eb and an independent Bb/F/D, and find the D tuning more useful, and generally prefer the independent arrangement because yes, I do get Db and Gb in 1st position.
There's a lot to know, but not much of it makes sense until you get one and play it. I'd personally get a Bach, Conn or King for $1500, take it to a brass tech and have him fix it up for $2-$300, and then play the spit out of it.
Best of luck.
