I'd recommend looking at costs. your build cost is going to be well over other homes because of the fixed costs like slab, Hvac, roof, etc. check the market drivers and weight the benefits of short term versus long term. long term is no furniture and help pays mortgage but at this size it doesn't really cover everything. we didn't find a lot of economies of scale in single family detached. can you build more than one unit?

You get what you pay for with a bigger firm that is cheaper.

Quote from @Jake Handler: For anyone that has been a buyer or seller of a seller finance property, what method of payment and tracking did you use for buyer to pay seller monthly? 

Anyone over 18 fills out has to furnish their social security and driver's license numbers, and pays a fee for a credit / background check.

And, even then, hitting your rate target would still be extremely challenging unless you were paying points up front, in which case it would probably make more sense to just try to put in a bigger down payment (or pay down the balance if refi) instead and get better terms naturally offered at the lower LTV.

Also, the credits are "use them or lose them" - meaning, if you pay $12k a month for a 100,000 skips, then you have to use them all that month.

However, with the rates and conditions of any triplexes and fourplexes we could find (we did find one, but it's in the Pendleton Heights suburb of KCMO, so we were worried about it being too dangerous of an area), we were wondering if it's worth it to buy a duplex and rent out the other half to help pay the mortgage, or if it's more worthwhile to rent an apartment and try to save up to buy a residential or commercial MFH later?

Never in the history of the Federal Reserve have prices gone down for any notable period of time.

Do they pay within 5 days of the first of the month (in some states that is not late)? 

If you do have a TX LLC just to hold a TX property, you still have to register in CA as a foreign LLC, which still means paying the CA LLC tax.