PBL, especially if you have a lot of students that struggle with mathematics, is definitely something that you will have to design carefully; ordinary project based approaches can sometimes exacerbate already existing difficulties. At the same time, it is important not to lower the rigor or expectations for those who struggle, and so the standard recommendation that folks might offer of differentiated instruction, where you give instruction at different levels and with different emphases to different students, is probably ill-advised, particularly if you are hoping to have all students work together on projects.
Having reviewed the literature in this area, I think there is one study that really stands out as an example of what you would ideally be striving for. Bottge et. al (2007) demonstrated the effectiveness of a method called anchored instruction with students with learning disabilities. In anchored instruction, students are presented with a video case (or "anchor") that serves as a source of data for a project-based lesson. More basic mathematical skills are presented to the students in the context of the problem, on an as-needed basis, rather than being taught prior to engaging in the project.
You do mention that you worry about your students being on different levels and trying to keep them together working on the same material. You might consider using an instructional system known as Complex Instruction (Cohen and Lotan, 2014). In Complex Instruction, teachers carefully design both intellectually rigorous and accessible tasks for students to work on in groups and then carefully observe (and sometimes intervene) to make sure that all students are actively participating.
Bottge, B. A., Rueda, E., LaRoque, P. T., Serlin, R. C., & Kwon, J. (2007). Integrating Reform-Oriented Math Instruction in Special Education Settings. Learning Disabilities Research & Practice, 22(2), 96–109. https://doi.org/10.1111/j.1540-5826.2007.00234.x
Cohen, E. G., & Lotan, R. A. (2014). Designing Groupwork: Strategies for the Heterogeneous Classroom Third Edition. Teachers College Press.