Purchasing your parents home is not the same as purchasing a home from a unrelated 3rd person. Assuming you are your parents' heir, the tax consequences of an poorly planned transaction can create issues for both you and your parents.

You must consult with either a tax accountant, a estate planner, or an estate or tax attorney to discuss various ways this property can be passed to you, while imparting tax benefits to both you and your parents.

You make no mention if your parents are going to sell you the house at less than fair market value. If they do, the difference between the sales prices and FMV can be construed as a gift which would be counted against your parents' allowable gift and estate exemptions total ($12000 per year) and creating a taxable liability. Also, a discounted sale price would mean that you would pay more capital gains when you sell the house later.

Your parents have the option of selling you the house at FMV, but carrying the note at a discounted rate. I forget the name of this rate, but it is rate that is set by the IRS and it is below current mortgate rates. You must make payments to your parents as per the contract, but your interest is deductible as in a standard mortgage. Your capital gains upon sale would be reduced because you are paying FMV instead of a gifted price. If your parents' original intentions were to gift you the home, they could reduce your payments by gifting you in cash yearly up to their $12000 gift limit, thus avoiding a gift tax.

But the bottom line here is that you are your parents' heir. Take the steps needed to protect your parents and yourself. And if you are going to get into a contract with your GF, you're better off marrying her first or make her pay rent.