DO NOT buy them a new one, just any old used one that fits in the space. 

I'm considering finding a single family home that would fit our family down the road, and buying it now to use as a rental until we're ready to relocate.

The standard size from Home depot will not fit.

Has a form of egress - door to the outside or windows that can fit a person.

One of the units will be our primary residence until the new house is ready.I’ve started speaking with our realtor and have done my own browsing and it seems like multi unit homes are hard to come by so it will likely be tough to find a good prospect where the numbers make sense, but we’ll be prepared to jump if we find a good fit. 

If you really like this style, maybe look for an A-frame in different location that will fit your investment goals.  

@Kit CreagerI managed to get one finished up a couple months ago, but real deals that fit the BRRRR model are a challenge to find.

If you look closely,  there are pockets especially in the fort worth area that may fit your buy your criteria.Another option will be to consider off market deals.Assuming finding off market deals is not your forte, Upon running your numbers on the MLS, don't be scared to make an offer that actually works for you and the spouse.

@Moaaz Malik investing in a market that doesn't fit your investment objective isn't risky.

There are several reputable turnkey companies that could be a good fit.