Great deal on a used Flex .30 at AoA

Early model, no Cobra regulator but your right, 1k cheaper than new is a pretty good deal. Too bad Jim G. doesn’t have any Cobra reg. blocks available. Easy to convert to .25 or .22, Wicked Air Rifles can make you a barrel in the caliber of your choice. Magazines are also available. Nothings cheap though. I would buy it if I had the finances. American Made Custom, low production air rifle. 
 
If this is one of the old models with the WAR valve and heavy spring and hammer, you may like it just as it is without the new Cobra equipment. My first Flex was an original model I later upgraded. The original models in .30 can run from 70fps all the way up to 114fpe or so just by hammer adjustment. You can get about 24 good tight shots at 80fpe or 8-10 shots at 114fpe. That makes it a good all purpose hunter if you don't need max shot count at a lower power (which is the advantage the Cobra equipment gives you). 

You'd want to add a $20 or less Law Tactical Chinese clone folding stock off of Aliexpress. That will let you access the hammer spring adjuster by just folding the stock. Otherwise you have to remove the AR stock every time you want to adjust it. 

You'd also want to look into getting a barrel tensioner from Jim that is 1/2UNF. Then you can add a better suppressor (although the old style suppressors with the heavy plastic baffles weren't bad). I run a DonnyFL Shogun on my "short" Flex (meaning its the old one with the 17" barrel) and its fantastic. 
 
The Cobra equipment, with the 17" barrel, will top you off around 90fpe or so. The newer Flexes run 20" barrel and my second Flex has a 24" barrel. Go Cobra only if you'd rather have a gun that does great at 70-80fpe like an Impact but doesn't have a lot of room to climb higher. The original idea being the Flex was it to be a "flexible" hunter, and its most flexible when its unregulated and using the original WAR parts from the first production guns. 

The only other way I'm aware of to get mondo power out of a Flex while regulating it is to run a Cothran valve in it. My long Flex has a Cothran valve. I run it on a 2100psi set point for 117fpe. I suspect the gun could do 200fpe on a higher set point. 
 
If, IF, I give in, I would be looking into what it would take to make it a .35 pellet shooter, 140fpe for one mag, 6 or so shots in as tight a spread as possible. I'm not convinced that cast > pellet for ranges inside 75yds which is where all my airgun hunting of any kind resides. For me, I want to have as few guns as possible to cover the most shooting scenarios as possible, from plinking in the basement to hunting. Right now that means a .177 Verminator and a .30 Bobcat. These are what I call compliment calibers, as in their effective ranges on the scale of shooting scenarios butt up to one another bu do not overlap. This basically necessitates an adjustable power small caliber.

Being that Florida and Alabama approved .30 for whitetail, (they're little down here) I hope that Georgia will follow suit soon. Bullfrog's ballistic gel experiments further convinced me a 75fpe .30 diabolo pellet will be as effective as a recurve bow with a simple fixed 2 blade broadhead out to 50 or so. Extrapolation aligns a 140fpe .35 diabolo pellet with a modern compound bow with more advanced broadheads, and again thanks to Bullfrog's, there is supporting video the .35 pellet effectiveness on whitetail. I choose to hunt whitetail with a compound for the same reason I would want to use a .35.

All that said to say, if I were to get it, it would be converted to a .35 and then I would have 3 guns that covered all my shooting needs. The problem is that .22 is the compliment caliber to the .35 and I would have to get another gun to complete the set as it were, cause them's the rules. So you see, rant aside, I really can't get it.
 
To make the power level you'd want, you'd probably need a Cothran valve. And if you go with the Cothran valve, you'd probably want to regulate it anyhow because otherwise Cothran power valves don't really shoot on a bell curve. Each shot is a bit less than the last unless you regulate it. So by the time you add a .357 barrel, a Cothran valve, and a regulator, you're coming close to the price of a new Flex anyhow. 

BUT... if you go that route, you would then have a gun that you could switch calibers with by simply popping the barrel and bolt probe out, and you could adjust the regulator up and down to match your caliber and power wants. You'd leave the Cothran valve in place across platforms and just trust the regulator to let you govern it. It won't be as efficient at say the Cobra valve would be at lower power levels, but it would still probably be efficient enough to still be a good .30 pellet shooter and maybe even a good .22 pellet shooter. I bet you could set the regulator down to a ridiculously low level to make good power for the .22. barrel.