It's probably a good idea to flush now because you don't know what the previous owner was feeding the plant.

Check the runoff pH and then flush with water adjusted to compensate for any problems. Tap water is fine.

The final part of the flush should be a weak solution of a complete grow fert with micros, at 1/4 of what the bottle recommends.

Drill extra holes in the sides of the pot if you are concerned that the plant is not using water fast enough to dry the soil properly between waterings. This will increase oxygen to the root zone and plant health WILL improve.