Because these so many variables in each situation it’s hard to say exactly.
Have you been through this course module yet -
This will give you all the things you need to think about around the data model and how to work up to a good one using the query editor.
Getting data from different data sources is very easy. There’s nothing difficult about it. You can bring in data from as many location as you want at the same time.
Around the consolidation, these just too many variable here. I would need to see a specific example.
The key thing to think about with project like this (and it seems you somewhat are) is how is the data model going to look.
One thing though that I sometimes do it just starting mocking it up and testing what works and what doesn’t. It’s really the only way when things get a little more complex.
Certainly run through that course module if you can, there’s plenty to learn in there.