Best small square baler?

Honestly I would phone the dealer on the particular baler for sale. Ask if knotter parts are available. Everything else a guy can work with. The 24T seems to have a a fair amount of aftermarket parts. How many bales do you want to make/pick? The last square I had was a 8550 case inline. Was quite happy with it and a guy might be able to pick one up for a bit more than some guys want for an obsolete side delivery.

I'm a New Holland guy myself, run a pair of older Hayliners, 276 and 278, parts no issue. Deere 336 and 346 were good ones as well. I wouldn't go after anything much older than those from in the 70s, it was at this time that most replaced the plunger slides with bearings, much better pickups and so on, actually much of those same features are still seen on the new ones today they had back then, however most early balers were not so equipped. Depending on what and how much you want to use it, but the larger models almost always have wider pickups, for example there is a foot difference between my two NH. Also larger models can make better bales in light swath conditions as most have longer bale case and other features. Added features like hydraulic tension and such aren't really needed, but are nice. The inline style like Beer mentioned some like yes but they were never that popular, I wouldn't have one simply because its hard enough on my neck to look back to one side, never mind straight back, but thats my opinion, its a personal choice.
As for can the older ones work, well just pounded out over 600 last nite in a bit less than a few hours with four loose ones and each of those was from twines breaking as they were coming out, I tend to run my tension fairly snug and around 60-70lbs as I need nice tight firm and square cornered bales other wise I run into issues with my stacker. For me for example, bale length is critical, I aim for 36-37 inches and the wagon is set it can handle up to 40 with no issues, but over that, now you have trouble. And what causes those over size is over loading the baler. Bale consistency is totally connected to how the baler is driven, over load it, bales get too tight and too long, always better to run them less than full, then when you hit a momentary thicker patch it can handle it without having to shift down just for a few feet. Variable swath density can be a pain as you need to keep changing ground speed.
The heart of a baler is the knotters, they are extremely sensitive and persnickety...kind of like setting points on the old style ignition, a few thou here or there makes a lot of difference lol. And guys like me who know how to fine tune them are getting few and far between. There are some books from NH that are good. Also to note, some older balers have probs running plastic twine, again a good reason to stay 70s and newer.

Now thats going back a few years...is that the 16X18 one? I have a 214T I used years ago, picked it up for $40 thirty years ago and baled thousands with it, still down at Spruce Grove at my old neighbors, one more thing that never got hauled up here yet lol, you'd think after twenty years I'd be done moving lol. Does that have the big slip clutch set up inside the flywheel itself? not the actual baler, thats on the main driveline. That was a slick set up on the 214, sure saved on shear bolts when you pushed it. That was my main baler till I got my mitts on the 276 which a friend found for me and traded me for hay, that baler was like new, always suspected because of where it originated, it somehow had been appropriated from the Enoch reserve band farm when that all collapsed years early. Paint wasn't even wore off the pickup bands. Admittedly, that 276 could almost bale circles around the old 214, but kept it going as back up and extra when I had a driver.

Anyway, found the book and thought this might help, closest I have for that one, do have the books for Gramps 14T, its sitting beside the other one, well it was last I seen it lol

Anything else I might be able to help with, ask.

So apparently my father-in-law has a Massey 12 with a thrower on it. I haven't seen it but price is right. Expect more questions next year from me...ha ha

Well those are a critter unto themselves, but if it will do the job and the price was right. Have one grown into some trees, wish I had pulled it out there twenty years ago when I moved here, been meaning to get it out of there, don't want to cut down all the trees, figured was easier to dismantle it instead. A friend robbed the drive shaft and slip clutch years ago for his 124. so was some interchangeability with the newer ones. Not a great pickup, but it depends on what you intend to do with it, narrow and widely spaced teeth, common in the very old style machines. Seems some guys swear by those Massey knotters, again different than any others. Keep it in mind, I don't think it did a whole lot, and when I knock it apart to move it, I won't be wrecking anything so lots of parts. If wasn't so much distance I'd try to manipulate it into a down payment on that intriguing lawn mower you got sitting in your shed lol

AB, the human encyclopedia...lol. Wish I had a memory half as good as yours!

I still have the lawnmower, and if I'm ever coming your way with an empty deck...and I remember...I'll be sure to get hold of you.