What does 50 + 7 uF mean on this capacitor?

It is a dual run capacitor: one section provides 50 uF for the compressor and the other 7 uF for the fan motor. Both share one can and one set of terminals.

Will it replace my old dual run capacitor?

Only if your old part is also a dual run capacitor with the same 50 uF and 7 uF ratings and a 250 VAC or higher rating. Match the capacitance and voltage exactly; a mismatch can damage the motor.

Is 250 VAC enough for a 240 V air conditioner?

It is rated for 250 VAC, which covers a 240 V system, but many OEM capacitors are rated 370 or 440 VAC for extra margin. A higher voltage rating is safe to substitute; a lower one is not.

Can I install it myself?

Capacitors store a charge, so discharge the old one before touching the terminals and disconnect power at the breaker. If you are not comfortable working on HVAC equipment, have a technician fit it.

Does it come with mounting hardware?

The product is the capacitor itself. Standard round capacitor straps or the unit's existing bracket hold it in place, so reuse the hardware from the old part.