Since you stated that you wanted something with lots of aftermarket parts, I don't think that the Ruger 10/22 can be beat in that category. What sort of modifications do you want to make?
Even stock, the 10/22 remains a good plinker. There are many better .22lr caliber rifles and carbines out there, but it's still a good value, fits your budget, and shouldn't disappoint. I'm happy with mine. It's stock, but is the "International" (Mannlicher-stocked) version that recently was discontinued; it wears a Williams Fire-Sight Set that works quite well.
Other than the Ruger, the various Marlin and Savage bolt models should fit your price range, as well as the Remington 597. However, I don't have any practical experience with those rifles, so I'll leave comments on those to others.