Gun for occasional shooting of possums and smaller

Trying to keep costs down, Crosman, Benjamin and others will all have a mid-power option to meet your needs.

Any upgrade or cost escalation should always be about a better trigger.

Ignore all the fantastic >1000 feet per second nonsense. 

.22 option in 700 - 800fps range is more than ample power for what you are proposing.

Gamo .22 Black Knight is my goto for pest control @ 20ft/lb.

Perhaps I was lucky and mine has performed without issue for several years.

Seems to be a few around scoped up for <$600.

You would want the Gamo 3-9x40 packaged scope.

Coming down to @15ft/lb would be Gamo shadow IGT series, less hitting power but better control and scope friendly. Shot placement will be more manageable.

From my posts you will note I am a big fan of the Sig ASP20.

Adding a durable scope and good mount will quickly total $1100 - $1400.

Fixed barrel options with under or side lever tend to sacrifice some power but accuracy improvement will reward every time.

Purchase of preowned quality brand can be good advice but lack of warranty and delivery issues can be a pain.

Whatever you choose, take your time and establish a repeatable technique. Get a solid pellet count in early and if you think something is amiss send it back as it will only get worse, the big suppliers just swap them out with few hassles.

Lots of news and pictures please, it is why we are here.

Very effective, lightweight but bit more of a work out.