If you are concerned with malware then do not purchase a device that haven’t received a software update in three years.

If you want a cheap iPhone that is supported — get an SE or used Xr / 11.

Just don’t film it and enjoy the show. You will watch that shaky, grainy “video” with terrible sound maybe once and will forget about it, but it will forever change your perception of the show, since you will be trying to film it and “not screw up” instead of actually enjoying it.