If you're 30 miles out from Houston, I'm guessing you're talking about somewhere like Richmond/Rosenburg/Mont Belvieu/Rosharon?
1) Are you planning on self-managing forever or giving it to property management in the future? I planned on self managing, but then I had to move away for work. I also discovered I'm a terrible property manager, and I turned it over to someone who hopefully knows how to run it a little better than I do. You force yourself into a hole if you don't account for property management upfront.
2) I don't think you can assume 3% housing price appreciation per year. A lot of the chatter I'm hearing is that winter is coming for a few reasons.
A) The fed is increasing interest rates which leads to lower housing prices (as peoples payments go up, they can afford less house).
B) Markets are cyclical, and we're close to the longest we've ever gone without a downturn. Even if prices don't go down and we don't enter a downturn, you can't assume that there will be sustained 3% growth. Nationwide, there hasn't been 3% inflation in nearly a decade. Will the deal still make money if you don't have appreciation, or a have lower rate of appreciation?
3) Rents may or may not go up. Even if they do, it may not be enough to cover the big capital expenditures. Your risk on this probably depends on how long you plan on holding these houses.
4) Have you accounted for all selling costs in your IRR? There's more than just the 6% seller fee. Taxes (on your capital gains from 3% appreciation per year), Title.
5) How are you doing your IRR calculations? It seems to me like you should be getting more than just a 12% IRR assuming 3% appreciation per year. Assuming you break even everywhere else, 3%/20% down payment gives you a 15% return. Between principle pay down, depreciation + mortgage interest on your taxes, cashflow, and everything else you should be getting more than just the basic 15% you're getting from appreciation and leverage.
There's some decent chance of an upside, but its risky to be counting on there being appreciation in the area. If the deal can't sustain itself as it currently is long-term, then I would stay away. There isn't enough potential upside (I would want more than 12% IRR for a speculative appreciation play) to take on the potential downside, but that's just me.