I think this is highly dependent on the industry and customer types. And while I think some agencies can be helpful, there is also often a meaningful fixed cost associated with them that can be a serious drag on margins until you start seeing significant sales growth from their work. I'd dig into your specific industry, see what others have said is most effective and spend time trying to implement some of this yourself in the beginning.

Depending on the size and sophistication of your acquisition, I believe many of the businesses you are likely to acquire are missing basic lead generation strategies via tools like Google Ads. There are many digital marketing agencies, or even freelancers via Upwork etc who could help you move the needle here quickly and cheaply. Agree with everything said above about the focus on learning and improving operations, but if you can increase and stabilize lead volume on day 1 without too much trouble, it gives you more leverage and control over every other decision you make. Happy to share some learnings from my own acquisition, if you're interested.