Have you seen the condition of the other residential unit that gets $900/month?
What is the average monthly income in that city and what is the average rent?
IDK anything about income property in the Caribbean (i'm an investor in Ohio)...but I just don't feel you have enough info to make an informed decision. Personally, I would get a couple of quotes from contractors for complete renovations. And I would talk to some property managers or agents to get more concrete ideas of what you can expect in terms of retail and residential rents. You have a lot of good questions but you don't seem to have any answers. I would need to go in with solid comps so i coulmd run the numbers. You need to be able to project what your costs and revenues will be to say if what the seller is asking is a fair price.
3b (with 3c tendencies) on modified CG