If you want a gun to last you forever, GSG's are not it.
You want something made out of steel, ruger MK 3, Smith Wesson 41 , Hi Standard supermatics, Smith Wesson model 17, ruger revolvers will all last for ever, if cleaned and oiled once in a will, The browning's are good also I hear good things about the S-W victor that have out now, don't like the look, but that is just me.
Lots of good euro target 22's out their, but mags can be a problem.
I have never seen the point to conversion units, you pay as much as a good gun, thou the CZ's are good
Every one need a quality 22 revolver, new ones will cost more than you want, but lots of good used ones out their.
and there are lots of good ruger MK 2, 3 , the MK 1 is a good gun also, but no hold open on the last shot and the early ones take a 9 round mag, later MK1's will take a MK 2 mag, 10 rds.
You can pick up good ones for 300-400.oo in blue, Browning Challengers are nice , but mags are hard to find.