The white spots are not powdery mildew. They are a mixture of pesticide residue and hard water stains from overhead watering in the grower's greenhouse. Powdery mildew is usually "fuzzy" and residue is flat on the leaf.
Either way wiping the leaves with a cloth is sufficient as a first stage. If it is powdery mildew it will return, residue will not.
The necrotic (dead) spots on the leaves are typical of root rot but can also happen if the plant has other types of fungus/virus/bacterial infection.
Given that you have repotted it the best solution is:
- wipe off the residue and see if it returns
- place the plant in high light
- remove all dead leaves and stems from the soil at the base of plant
- when the top inch of the soil has dried, apply water until it comes out the base of the pot
- do not let it sit in water
If the plant continues to deteriorate you should return it to the store as it may have been sold to you with a fungus/virus/bacteria. A healthy plant under good conditions can outgrow minor issues.