@Ethan Mace
1. Talk to your accountant about a 1031 exchange. It simply allows you to deffer taxes on any gain you would have by allowing you to transfer your basis from the old house to the new house. Your responsibility to pay back the bank for your old mortgage and tax treatment on the sale of the house are separate considerations. Lets say of the $100K, $80K is the banks and $20K is your down payment. Under the assumption you had an interest only loan and there are no closing costs for simplicity, $70K is now yours and the $80K is owed to the bank. Skipping all the steps around who can and can't hold this money, you now need to buy a house in a set period of time. The basis in your old house, $100K (less one year depreciation) will now be the basis in your new house, plus any amount above $150K which you paid for the house. Any amount paid under the $150K for the replacement house is taxable, as your will be forced to recognize the tax gain to the extent that your receive consideration (cash or debt reduction). Any gain is taxed at your normal tax rate to recapture depreciation you have taken as a deduction on the house, and as a capital gain thereafter.
Basically, a 1031 exchange is just a tax strategy which allows for deferred taxes. You can't take money out of a house with it, but rather trade up without paying any taxes on the gains you have made until you sell the replacement house. Think of it like using a equity line of credit to remodel your investment property, but instead buying a bigger one.
2. With respect to the loan type, the IRS doesn't care how you finance a house. They do care if it is owner occupied through, which I believe is a requirement for the FHA loan. You may be able to use it on the investment portion of a multi family though, i.e., half of your duplex is treated as investment and half is not.
3. How to ensure capital gains rather than losses? Invest in treasury stock instead of real estate. Your gain is derived from the risk premium. Do your homework and you have a better shot of doing well. To tie it in to #1, the purpose of a 1031 exchange is to defer yours gains (ordinary and capital).
4. Don't speculate on appreciation for your investment strategy. Your buying an income producing asset, so count on the income. If your not in it to hold, then make your money on the front end rather than hoping for some luck on the back end.
I hope that helps. I'm not an expert on tax law and have never used an FHA loan so take my understanding with a grain of salt.