From someone who went into a practice with similar staff situations....
You have to look at not just what the staff is getting for full compensation (pay + benefits) but why they are. If it's a situation where they're getting paid far more than the going rate simply because they've been working at the office for a long time and have gradually seen increases in their pay not entirely based on job performance, but also based on longevity, that's a far different situation than if their pay is based mainly on job performance.
Additionally you may also find that the skill set that the existing staff has may not be what you, as the new owner/boss, are looking for, and that could hinder your vision for the practice, and that would be a problem.
You will have to probably do far more "coaching" of the existing staff than you would expect to if how YOU want to practice is different than how they are used to practicing. That may very well result in YOU having to make some changes, changes that they may not initially like because it's not the way that the previous owner did things. Some staff are like this, others. even ones that have been there for years, may be more than welcome to changes because they may not of been happy with how the previous owner did things. You likely won't know how this will all work out for 3 to 6 months, maybe even a year. That is the challenges of being the boss.
As for the existing practice patient base. Some will stay, some will go. that's just a fact of life. Implementing what it sounds like is a NEEDED external market program will likely fall more upon how the existing staff can handle things, and how you want to go about it. Don't be afraid to try new things for the practice, and also don't be afraid to admit that after trying something that may end up not working, that it's time to reevaluate if it really is working, and if it isn't make changes to it