If you are buying for investment purposes your FTB status is irrelevant for stamp duty and, I think, mortgage interest relief purposes. If you buy an investment property, however, you lose your FTB status. I've personally decided it's not worth it, but that's my decision. FTB is really only relevant for owner occupier.
Do you intend to live in the property at all, because if so, surely the rental market should be your secondary concern?
With the DAFT site you can get an impression of how many properties are for rent in your area and how long it is taking to shift them, along with current expected rental. If I were in your position, that's where I'd start.
If you're looking to rent-a-room, then that's slightly different. Look at houseshares on daft then to gauge how many rooms are available in the area.