The issue is usually with the inability to get comparables on the appraisal, rather than strict FHA guidelines. Almost all lenders have overlays today; however, so there may be issues with the separate dwelling. You could have a non-occupant co-borrower in either case; however, FHA is the only way you can use their income to qualify. Again, this may be lender specific. Very doubtful that any rental income from the additional unit could be used unless you 1). own other properties and have a history of experience as a landlord and/or 2). you put in exceess of 25% down. If FHA works, a down payment of 3.5% would be required. If you go conventional, the down payment would be 5% to 10% minimum; however, you would have to qualify for Private Mortgage Insurance and their guidelines are more restrictive than lending (credit scores and reserves, in particular).
As with most finance scenarios, it is very difficult to think of any questions you MIGHT be missing until going through a full interview process to know the particulars of your situation. It really is best to talk with a mortgage professional personally. I know of one, btw. :)