Collecting within established areas of medicine and surgery can be competitive. So it is a idea to look for your own niche and this will be guided by your specific interests and likely as not by the ubiquitous constraints of size and expense.
However, deciding which area to settle on is potentially fraught with danger. You can pick up a room full of 20th century medical or scientific antiques for next to nothing … but with good reason.
Good advice to any prospective collector would be to focus on a narrow area of interest, bone up on it and then stay only within your level of expertise, at all costs.