Since PCP air guns require special equipment, I'd suggest you work backwards starting with accessories instead of looking at the gun. Start with how you plan to fill your air gun. Will it be by pump or will it be by bottle? Air pumps will run between $160 - $500 based on what you purchase while bottles can run from $125 (used scuba tank with 3,000 psi rating) to carbon fiber tanks around $800 (4500 psi rating). Both have their pro's and con's and both may require additional equipment to fill/be filled. I don't recommend using an air pump on any air gun that has over a 3,000 psi rating.
Next, look at your optics. Very few PCP's will come with sights so if you don't have a spare scope and several sets of rings, you again have additional expenses. Some air guns have a 11 mm rail while others will have a Weaver/Picantary style. Also, you may have to use high mounts instead of medium mounts due to the air gun's magazine. This basically means that just because you have a scope and rails, it doesn't mean you still won't need to get additional mounts or rail adapters.
Now you can look at the airgun itself and decide if you want brand new or gently used. For brand new air rifles under $600, you will be limited to several manufactures with Crosman probably the most recognized. They offer the Discovery for $300 (2,000 psi, single stage trigger, single shot), the Marauder for about $550 (3,000 psi, two stage trigger, multiple shot) and the Marauder Pistol for $400-$500 (based on camo or no camo, 3,000 psi, two stage tripper, multiple shot).
While I don't own a Discovery I do own the Marauder and Marauder Pistol. From empty, the Marauder takes between 250 - 350 pumps with the Benjamin Air Pump to fill and then approximately 75 pumps every two magazines to recharge. The Marauder Pistol takes about 120 - 145 pumps to fill (it has a much smaller air cylinder) and about 35 pumps every two magazines to fill.
Having said all this, if your absolute limit is $600 go with a Discovery combo or Marauder Pistol (if you have a scope/red dot sight). This will get you the Discovery (which has iron sights, a 11 mm dovetail rail, and is available in either .177 rated at 1,000 fps or .22 rated at 900 fps) and a Benjamin pump for under $500 while the Marauder Pistol (.22 only rated at 800 fps, 11 mm dovetail rail) will require you to purchase the Benjamin pump separately (they do offer combos at times) and come out closer to the $600 limit.
Note that with the Marauder Pistol, it does come with a replaceable stock or you can add an AR style stock with the RR Arms conversion kit. Also, the difference with the Woods Walker edition is that it is camouflaged and comes with a red dot scope which accounts for the extra $100 in the price.
If you are able to go a bit over the $600 budget, look at the Marauder synthetic stock combo kits. These will range from $780 to $805 based on caliber, but will include everything you need including air gun, air pump, scope, rings, extra magazine (which is nice especially when the original breaks). (Note, prices and combo kits listed are from Pyramyd Air).
