Depends on your goals, your timeline, the amount of money you are planning on investing, the cash flow it generates, if you are married or single, your annual income, your tolerance for "tenants, turmoil & toilets", and a couple of other things.

I got married to my beautiful wife and recently separated from the service.

Got married and for a while wife didn't want to do RE because her father "suffered" through it for years.

Live in is a good strategy especially if you will live there for 2 years or longer (section 121 of the Tax code) you will not pay tax on gains up to $250K single or $500K married, keeping that in mind markets change, so consider where we are and where you think you will be 2 years from now. also consider if you are there less than 2 years the gains are taxable at a very high rate about 1/3 or more of net profit for most people.

I got married in my own back yard, which worked out very well.

All very valuable, none more so than learning how to avoid marrying social climbing females!

Also, we are not married, so can my girlfriend legally buy it?

We are not married... is there any way to do this legally?

He got married, moved out, costs have gone up, and he made a few adjustments, rented out the 4th bedroom and continues to hold it.

I am married and although my wife is onboard with house hacking and growing our wealth she told me a backyard will be a must have so we're leaning toward duplexes.