Cash on Cash return seems low at ~3.4% which includes prepaids for property taxes.

Many today are requiring 3-6 months of prepaid interest.

My initial analysis showed that this should cash flow $300 per month with our initial investment of $21,000 (25% downpayment plus closing costs and prepaids).

.  ~$11,000in closing costs: 4.4% points ($4,500)$1500 underwriting and processing$700 appraisal and credit report$1300 in title related charges$1000 in taxes$1300 in pre-paid$800 in escrow

William Bannister is absolutely correct.Here's some examples of tax sale properties that I've seen:bottom of man-made lakeside of cliff 50 feet tallArmy bombing rangeExpired land lease for river front cottage totalled by flood 30 years agoExpired lease for TV tower pre-cable, that was already removed from propertyLand only with building owned by another party who had pre-paid their lease for 30 years, so if you bought you would start collecting rent in 3 decades.super fund toxic waste siteAbandoned gas station with leaking under ground tanksAbandoned steel millhouse with only 3 walls as the rear wall collasped into back yardhouse on city demolistion listStrip of land 2 feet wide, left over from road widening projectLandlocked residual land from highway projectAbandoned QuarryAbandoned junk yard, various colored ground stains from antifreeze, gasoline, oil, automitive fluids and metal parts.Slag pileChromnium factoryAbandoned building filled with 55 gallon drums of unknown liquidsRavinesWetlandsFlood waysDrainage ditchesSite with endangered speciesGarbage dumpHave a nice day!

Your equation doesn't look like it includes closing costs either which typically would cover prepaid taxes, insurance, etc. 

Or, if you really want to be property - eliminate all prorations and prepaids from the settlement statement. 

It took 2 DAYS to get customer service to call me back, and everytime I have been on hold with them before this issue, it was 30-60 minutes on speaker phone.Anyone can go get a pre-paid card, put $5 on it, and use it on their website, show up at your place, and claim you didn't allow access, and then get a nicer place, with you being stuck with $1000's in relocation fees.

Here is the wording from Fannie Mae: "The new loan amount can be no more than the actual documented amount of the borrower's initial investment in purchasing the property plus the financing of closing costs, prepaid fees, and points on the new mortgage loan (subject to the maximum LTV, CLTV, and HCLTV ratios for the cash-out transaction based on the current appraised value)."

Here is the thing You probably have about $2200 or so in closing costs from the title and attorney or escrow$1800 or so pre-paids,  taxes insurance etc.There usually is an underwriting fee from the lender of about $1000 and an appraisal fee.Keep in mind you will need a good faith deposit and if you are not qualified to inspect the house, you need (not required but very important) to pay an inspector, on this house probably $350.Then the question is, will there be points to pay to buy down the rate, or will you accept a higher rate and get a credit from the lender.