Well that is a simple mathematics, if your inbound leads/ inquiries are increasing every month. And counting upon the new customers per month acquired in your business model.

The question is for you about how much ROI ( Return on Investment ) you want in a month. As it is all about investment first by keeping in mind about the ROI. There is no minimum or maximum limit can be defined in terms of marketing budget for your business.

2 What should be my minimum budget?

3 Do I need to spend extra on offline marketing too apart from my online marketing?

No matter you are having an B2B or B2C business model, in todays digital era if you aren't spending or having some budget. Frankly speaking you are standing nowhere in the competition.