the 883 is a wonderful 22mag, especially if it is wood stock, however if it is a synth stock, you should get a roll of aluminum tape, and shim up the sides and bottom of the action a bit, to keep it stable, while you are pulling the trigger rearward. As for the price, you positively stole if it is even in just working condition; a fair condition 22 mag would normally go for 150 bucks all day, a nice shape one should bring 200. A 783 marlin is one of 2 marlins more valuable, the most valuable ones are the ones that look like garands or 30 carbines.
It should also be an outstanding shooter; so make sure you try a variety of 22 mag rounds, to find out which it likes best.
You picked a fine, lifetime type rifle there, is what you did...