I have a Marauder in .22, and for the money, it's a great platform. Modifications are endless because so many of them are out there. Trigger is good, accuracy is excellent out to 75 yards or so, the .25 should easily give 100 yards accuracy.
The downside, it's a $540 gun retail. If you use the AGNATION code above and order directly from Crosman, and order on Friday (Free Shipping on Fridays), it's $405. That gets you the gun. Now, how do you put air in the gun?
You either need an air tank which will run a minimum of $350 or a hand pump which runs around $150 for the Crosman (25% discount and such from Crosman). That puts you around $550.
I hate pumping, and you will too. It's about 100 strokes to fill the gun from 1800 to 2800 PSI. Those last 20 strokes are serious work compressing 2500 PSI and up, so a tank is seriously a better option.
and you'll need Optics, as the Marauder has no sites.
A better low end choice might be either a Fortitude and pump or Maximus and pump. Watch the AEAC reviews for both before buying.
Fortitude - Airgun Depot has the gun, pump and scope combo for $489, but it's only a .22. Throws the Crosman pellets at a bit over 700 fps in .22 and gets 60 shots per fill compared to the 16 a .25 Marauder is rated for. Power is quite a bit less, but still sufficient for squirrels.
Maximus - really hand pump friendly with a max fill of 2000 PSI instead of the 3000 PSI on about everything else. Much easier to pump. Maximus kit runs around $400, but again, it's only a .22, not a .25.
Buying a .25 with an air fill system and scope will be incredibly difficult for $500.
Lastly, a .22 is plenty for a squirrel at 50 yards, .25 could be considered overkill (pun intended)
good luck, let us know what you pick.