Mr. Heywood has a point regarding resort areas. Connecting this to modern day resorts, when these things pop up, the prices of everyday things are inflated. This results in the residents of the area not being able to afford to live there and become impoverished.

Here is a map of Sussex for reference. https://www.visitsoutheastengland.com/places-to-visit/sussex/map

Does this refer to a surgeon's assistant?

In this chapter, we are introduced to the Parkers who are from Sanditon and are traveling. There seems to be heavy focus on health, probably due to the fact Austen was sick at this point in her life.