Is it possible to upgrade C2950-i6q4L2-m ver. 12.1 Catalyst IOS to 15.0 or higher and if yes... is there any recommended version with less issues? Would c2950 hardware supports 15.0 or higher?
I am sorry to disappoint you.
The latest IOS version for Catalyst 2950 switches is 12.1.22-EA14 from November 2010. Since that time, no newer IOS versions for this platform have been released, and this switch platform is itself End-of-Sale for quite some time, meaning that new IOSes won't be coming. IOSes for switches are not interchangeable, meaning that a 15.x IOS from, say, 2960 cannot be used on 2950, and IOSes for the 2950 have stopped being developed almost 5 years ago.
We have to face it... the 2950 lasted for an extremely long time and are still widely used but ultimately, this line of Catalyst switches has been superseded long ago.