I think you've thought it out well... That FHA MIP hurts when you've already paid down the balance significantly.
What's the difference in rates between a conforming, 20% down loan and the ARM? I *loathe* ARMs but in this market, I don't think you'd have trouble selling and honestly, with the income you'll have, you could very easily pay the entire mortgage by the time the ARM is up. Put in your mortgage debt at one of my favorite sites,
www.whatsthecost.com and see how extra payments could pay it off quickly.
If the purchase price is $150K with $7500 down (that's 5%, right?), it leaves a mortgage debt of $142,500. That's a payment of $722.03/month, if the ARM's interest rate is 4.5%. Plus, you'd still pay PMI of about $50/month until hitting 78% LTsaleV. (FHA would be 3.5% down at 5% so a mortgage of 145,250 with a payment of $779.73. Plus about the same PMI, in the form of the MIP.)
If you paid an extra $1000/month on the principal, the house would be paid off in 8 years, 4 months. At the 7-year point, the balance would be $24,814.19. And I doubt the house will be worth less than that.
lol - in this market area, houses don't shoot up in value but unless they're in the ghetto, they certainly don't go down. That would be a tidy sum to take to your next home/life adventure.
If you paid an extra $1500/month, the house would be paid off in 6 years, 2 months.
If you paid an extra $2000/month on the principal, the house would be paid off in just under 5 years. 58-59 months.