Input on what an appraiser bases value from when there are no comps

[Ben Russell]

We are looking at placing an offer in on a duplex in a very small town.  We would pay cash (to buy at a discount), then do a cash out refi. We need it to appraise for 250k.  It's fully remodeled, with current leases in place for 2k a month.

My worry is making sure we will hit our estimated value with the appraisal for the refi.  This is a very small area with a lot of mobile homes so the PPsq is fairly low.  The only true comp is another duplex that sold off market for 150k, so there are no pictures or a way to really determine condition.  Just based on the 150k comp and the PPsq in the area, we will not hit the required value for the refi.   

How much will the rental income effect the appraisal?  How will an appraiser look at a unique property for the area that has very few comps?

Thanks