I can only echo what others have said.
The current 50cc saw is a 550 MK II. It’s great with all the latest bells and whistles but expensive, a little heavy, and it actually ties you back into needing a dealer if you have to work on adjusting the autotune system- something I sense you’re trying to avoid.
The previous generation 550 XP I would skip- it had too many issues with hot-starting due to design. Not all of them were all bad, but I wouldn’t take a chance rolling the dice on one with so many other options out there.
Previous to the 5-series were the 3-series, and the best 50cc pro saw in that size was a 346 XP, which was a true 50cc saw, if it had the silver clutch cover. Sometimes referred to as NE, or new edition. In its day it was called a light saber. It was light, reliable, well designed and very powerful for its size, and made good gains if you went into porting it for more power. I would definitely recommend looking for one, but it will be hard to find or you’ll pay a lot for a good one.
On the same chassis is the 353, which is the pro-sumer piston and cylinder. Also a very good saw, just a touch behind in peak power, but nice wide power band and probably more torque than a 346 XP. You can also upgrade the top end if you want. And if you’re in the camp of buy-then-upgrade-parts, you can also look for a 351, or 350 which take upgrades very easily to become a 346 XP.
Before the silver clutch cover 346 XP was an orange clutch cover 346 XP “OE” (original edition) that was 46cc. Still a great saw, just a hair under 50cc. It’s identical to the NE version except for a different top end piston and cylinder. I wouldn’t walk away from a good one of these.
Before the 3-series and you’re going back into the 90’s with the 2-series, and the later model 254 XP is where it’s at. It’s better build than the 3-series, simpler, but a little stocky. Its about the same size as a current 550xp MK II, but it’s actually 54cc so it’s got great power to weight. Parts are getting hard to find, so if you’re looking for one you should 1) make sure you buy something working and complete, or 2) have a lot of spare parts and or parts hoarding friends, or 3) be a collector.
The early 2-series 254 SE is even simpler in design : rubber mounts, not spring antivib, no ‘air injection’ centrifugal pre-filter, etc. These are good saws but parts availability means it’s hard to recommend. I’m sure there are some great examples tucked away in garages and they may hit eBay in good shape, so they can still be a good buy. But keeping them going is getting harder.
Before the 2-series was the 1-series in the 80’s, and I would just skip the 154 SE unless you’re like me and want to see where it all began.
Also, in the 2-series I would look at a 257, which has a little more power than the 254XP, but wasn’t the factory hot rod the 262XP was. It’s a ‘non pro’ non XP model that was positioned between two XP models at 57cc.
And in the 3-series I would look at the 357XP (which was the actual successor to the 254XP; the 346XP OE was a replacement to the 242XP), but was often poo-poo’ed by people expecting a 60cc replacement to the 262XP. The 357XP is well into the 50-cc class at 56cc. Second to the 357XP was the non-XP 359 model. Like the 353 to the 346XP, it’s down on peak power but there is actually slightly more displacement in a 359 than a 357XP, with a nice wide power and and lots of torque, at just shy of 60cc.
