Machine...
Im in about the same situation as you.
I bought a house in May.... g/f moved in with me. main difference is that she is in law school so has very little income right now. since we do not have a contract drawn up, what we do is I pay the mortgage and she covers the electric bill (again, low income on her part)... She also covers most trips to the grocery store and dogfood. In case anything happens between us, we can go our separate ways and she didn't input anything into the house.
What I would suggest is that you buy the house, then workup a general RENT agreement with the g/f. You can either get a lawyer or you can find a generic rent agreement online. This will protect you in case of a split up. Otherwise, she can contest that her money went towards equity and now she wants her piece of the pie. not pretty.
What can you afford? man there is no 1 answer to that. I would log into a site like Mint.com or something that allows you to track every penny you spend for the month. Add in emergency expenses (car repair), house repair ($100-$400 monthly depending on the issue), and other unforeseen issues (wedding, honeymoon, etc.) and then determine how much you (BY YOURSELF) can afford. Do not factor in the g/f's money or that of a roomate. You need to plan for worst case scenario.
I was lucky with my purchase bc I love the house, but the neighbors really make the neighborhood. If you find a property that you like, make sure you take an hour to go door-to-door to introduce yourself BEFORE you are committed to the property. You will find out what kind of neighbors you have, the pro's and con's of the neighborhood, and even some background info on your property that would not show up in an inspection.
The two people you need to know right now are a good Realtor and a great Mortgage broker. most mortgage brokers are not your friend at all ( i was lucky to have a friend in the industry so i got a solid deal). Also, some realtors will recommend brokers, but they might also get a kick-back from the broker. You can shop around to 2-3 brokers and weigh their deals and offers. Since you are not selling a home, a Realtor is FREE to you and will be a great asset.
Dont forget to research the other costs beyond the mortgage. In south florida, your costs (insurance, tax, PMI, etc) can actually cost more then the actual mortgage. In my case, about 53% of my monthly payment is interest + equity, the other 47% is tax, insurance, PMI (only $70 a month), and extra for the escrow account.
There are a lot of desperate people out there, so take your time and dont settle. You have the upper hand in this game.