IPO,
Thanks for the sharing of the data. You probably already thought about the following, but anyway below are just some of my quick thoughts:
There are many variables in your analysis, which can affect the result quite dramatically.
a) a year from now when you purchase, the 30 year interest rate could just be the same as today which is in the 6 to 6.5% range. If this does happens, your NPV will be reduced due to lower mortgage payments.
b) use a 5% discount rate for the NPV calculation. For the purpose of this calculation, I believe the appropriate discount rate should be the rate of return for your family's overall investment portfolio ( which is your alternative investment for the cash flow). If 5% is what you are getting, then it is fine. I have no idea what the s&P500 index will produce on average for the next 18 years, but 5% over a period of 18 years sounds relatively conservative to me. If 8% is used in your NPV calculation, your NPV will be lower by a lot.
c) staying in the house for 18 years. Most people in the early 30s don't stay in the same house for 18 years due to many reasons. You might well be the exception, but again "never say never". Your NPV will be lowered if stay there shorter. Then, if you do stay there for 18 years, you might have an opportunity to refinance your 7.5% mortgage rate, which again will lower your NPV.
Any combination of (a), (b) and (c) above will change your result quite a bit. Due to the uncertainty of many variables in an excises like this, often it is more an "art" then science. I tend to look at those analysis as "sanity" check, then strict go-bys. In your case here, personally I will draw the following conclusion based on your analysis: if you believe the type of house you want to buy will drop 20% next year, your cost / benefit is well justified. 10% range, you are probably safe to at least break-even. If it drops no more than 10%, then it all depends.
I can't leave this topic without echoing it might seem to some of you as my theme on this kind of topic: IMO, it is NOT all about numbers when making a rent vs. buy decision on your primary home as long as your can afford buying the house you want. $50K NPV saving (which is uncertain anyway) over a 18 year period doesn't sound like a lot of money to me when buying a $1 million home. A better and sure way to save this kind of money (and more) is to buy inexpensive cars (i.e. toyota vs. lexus, honda vs. accura, etc) in your life time.