I have 12 years experience in ERP solutions like SAP and various others and get this type of question a lot from SME's.
I your case you want to provide implementation services and ultimately sign a retainer support contract for 36 months.
There are a few good ones out there, start with https://www.odoo.com/ , they have a good system that can grow with a business.

I have first hand experience with that challenge having developed and sold applications and SaaS solutions for 30+ years and been approached by system integrators searching for partnerships many times. Application providers want answers to the following questions: "What does the integrator bring to the table other than custom development expertise? Does the integrator have experience implementing applications like mine? How can they improve my client's chances of success? Do they want to become a competitor?" My recommendation is to focus on one type of business challenge in one industry at a time, become knowledgeable about the unique requirements in each industry then develop tools that will accelerate implementation, lower administration costs and/or improve the effectiveness of the applications. For example, data curation is critical for most enterprise applications so curation tools must be purchased or developed. Your sales team must thoroughly understand the industry's requirements, demonstrate the tools and explain how you can enhance the application provider's business. My expertise is in developing and selling enterprise marketing applications but I am sure that I could move to the other side of the table and help you create long lasting, mutually beneficial partnerships that will grow your business. Happy to follow-up.