Hi Eric
Your questions are some great ones to ask as a first time buyer and buying a duplex is a great way to help offset your mortgage payment, so congrats!!
Let me address each question for you as I have a lot of experience with rehab loans like the FHA 203K and the Fannie Mae HomeStyle.
1. The 203k will allow for peeling paint or any other cosmetic issues as such. In fact there really isnt much you cannot fix with a 203K. Just because it is an FHA product doesn't mean the home has to be in excellent condition.
2. This is tricky, while FHA does allow for what is called "self-help" there are not many lenders who will allow for it. That said I would not plan to do any work yourself in the grand scheme but if you have the right contractor you may be able to work with him to help shave your budget.
3. Yes the HomeStyle requires 15% with a duplex and essentially has no restrictions on what you do to the property. In contrast to the 203k which requires 3.5% down but does have some restrictions. Those restrictions are going to be more geared towards luxury items like putting in a pool or a sauna.
4. The 203K would be your best and only option for the low down-payment renovation loan.
Any other questions you have please feel free to reach out.