First, congrats! You are in a great place to start.
Second, the real question here is for you: do you envision your path to FIRE as coming through investments in the stock market, or are you planning to invest in real estate that you rent out to others. Both paths lead to success, but it affects the decision you make now.
If you want to be landlords, then buying a duplex is a great way to start, if you can find the right deal where the tenant effectively pays your mortgage. You can live in it, fix it up, then when you're ready either buy another rental or rent out both sides and use the proceeds to repeat the process somewhere else.
If you don't plan to be landlords, then you need to think of "buying a house" as a consumption choice, not an "investment." That means that you need to get your savings on track for your FIRE date
before you upgrade your standard of living. And that means maxing out your 401(k)s, IRAs, HSAs, etc., as laid out in the Investment Order sticky, before doing anything else. Why? Three primary reasons:
1. These accounts are tax-deductible. Minimizing the tax you pay now will allow you to invest more for the same hit to your budget. Which is important because . . .
2. The power of compounding. The rule of 72 says that if you get about a 7% average return, every dollar you put in today will double in a decade. So $10K now becomes $20K in 2028, $40K in 2038, and $80K in 2048. But note that if you wait 10 years to start investing, at the end of that same 30 years, you have only
half that amount -- you lose a full doubling. So the dollar you invest today is more valuable than the dollar you invest tomorrow, and the dollar you invest tomorrow is more valuable than the dollar you invest the day after tomorrow. And finally . . .
3. Those tax-deferred options are use-it-or-lose-it. If you don't max out your 401(k) this year, you can't "make up for it" by contributing more next year.
So in general, if you're on the "live off stock market investments" path, I would start with this post:
http://www.mrmoneymustache.com/2012/01/13/the-shockingly-simple-math-behind-early-retirement/. Figure out where you want to be on the chart, and then figure out how to put the required amount in investments every month. Then live on the rest. If that leaves you extra cash to invest, by all means look into buying a house -- especially if you can get a good deal on one, have a tenant that helps with the cost, etc. But get your savings ducks in a row before moving on to lifestyle decisions.*
*The only caveat here would be if buying a duplex actually allows you to reduce your net living expenses because the tenant pays so much rent -- if you can find that sweetheart of a deal, you could move that up on the priority list, as long as you then invest the extra savings. But even there, I'd still recommend maxing out your HSA and 401(k) to the match first -- and when you're doing the math, don't forget the extra costs of repairs, maintenance, power/water bills, insurance, etc. etc. etc.