Hi Bryan,
While I'm not the most qualified person to dispense any advice, I do own a number of apartment buildings, including a 12 unit, and I often wonder just like you where it's best to spend money to attract good residents. From my combined experience, it seems that a little exterior work can go a long way, as well as some interior.
First, consider hiring a pro photographer and get some good photos. Maybe pay them a little extra to retouch photos for you. I normally pay photographers $200 per session. Some of them can provide you with aerial photography and even video. Some candidates would be happy to view videos of the place.
Second, it pays to keep the place clean. Whether you, your manager or one of the residents can keep the place clean, it would be a boon.
Third, for exterior updating, painting the place could be expensive, so perhaps touch-ups would be a first step to start, and continue it as a work in progress. Maybe a little bit of landscaping. You may be able to get someone on Craigslist who needs a little extra money and can make the place a bit nicer. See if anything stands out, like broken windows, broken handrails, staircase etc, that may need attention. People may get a bad vibe if they see a place looking to run down.
Fourth, since you have multiple vacancies, start in small steps. Consider starting with the best vacant unit you currently have and see what you can afford to update. Replacing a vanity isn't all that expensive. Refinishing a bathtub shouldn't be either. See what needs updating in the kitchen. Fix holes in walls and dents in flooring. Give it a nice new coat of paint and pick some nice colors. Another option you may consider, if you have an occupied unit in great shape, offer the resident a discount to downgrade. They will move to a unit in lesser condition for a reduction in rent and you will have a unit in much better shape ready to go.
Over time, keep making improvements. In a year or two, if you keep at it, you'll come a long way. If on the other and you see that this building is not performing, perhaps consider selling. Would be much easier to sell if the building is not entirely distressed.