I have 4 properties in Norman, my brother has 3, and my parents have one. All SF houses. We've noticed increased rental supply with all the new development in town. We've managed to keep rents mostly steady, but in the past we were able to increase the rents yearly or at tenant turnover. I did have to lower the rent significantly on one unit when it came vacant last fall in order to fill it.
We're past the prime summer rental season, so even if you closed quickly on the duplex, it would be harder to get a tenant at this time of year, and you might have to offer a lower price to do so.
I also think that your figures of 5% for vacancy and 10% for maintenance are low. I budget 10% for vacancy and 20% for maintenance/capital improvements. Of course, my actual expenses vary from year to year.
I noticed that you mentioned 10% budgeted for property maintenance though...do you not manage the other side of your (current) duplex yourself? Self-management isn't for everyone, but if you are just managing the other side of a duplex you live in, it would be a pretty easy way to save some $ and get that per-door cash-flow higher.
I'd say what it boils down to is: how good a deal is this? Is the increased income going to be worth the extra time and gas money you spend on a longer commute?
Finally--in general I expect rents in Norman to stay steady over the next few years. I'm not expecting to be able to increase rents for a while.