I find artificial horizons to be almost useless, but voltage and speed are certainly nice. Voltage is easy, speed is a bit more complex. You need something with GPS and if youre going that far, For any serious dedicated FPV craft, id look into getting a flight controller. This is a whole new topic but i would suggest one that can run ardupilot.
If all you want to do is buzz around at close to med range id say just go with a runcam that displays voltage and time and keep it simple. If you're looking to get to the next level adventure id get a flight controller. From there your options to transmit telemetry data are endless.
In fact, id suggest that even in a simple park flier FPV setup id go with at least a ZOHD kopilot lite and not worry about telemetry yet. Its a nice bit of security to be able to go into stabilize mode when you need to and the RTH feature can really save your bacon!
Check out the 2:10 mark for the ZOHD plane and 9:30 for a plane running pixhawk: