Vertical blue stripes on iMac screen

Likely either a failed GPU or display. At that age, issues like that are to be expected. As mentioned the computer is obsolete (will not run current versions of Mac OS) and vintage (parts are difficult to locate) . Considering the age and cost of repair would be excessive it makes sense to upgrade to a new Mac or simply live with the issue.

Hardware problems. Unfortunately your iMac is considered obsolete and can no longer be serviced by Apple or an Apple Authorized Service Provider.