If they think your intent was to live in your home only 6 months, they may go after you. 

Hi Thu - Congrats on your first STR purchase—so exciting!

The property that you sell and then purchase through a 1031 Exchange must be held for rental, investment (capital appreciation), or used in a business. 

Purchased in 2015 for $57k seller financed.

So, you can do a land contract for 12 months, which should allow lender to credit you full value of the land vs the purchase price, but the loan will pay off the land contract, so the lender maintains first lien position.Can't help you with the rest of your question, but you should be able to search for "CPA" here on BP to find experts to reach out to.

I'd also like to try to purchase with a land trust or my LLC. 

Quote from @Victor Yang: If I gift a house, what amount will go towards the lifetime gift limit, the amount i purchased the house for, or the current FMV?

With this being your first purchase, you may want to consider a property that's not being sold at auction. 

Understand Realtor (Agent) Fees- In Germany, real estate agent (Makler) fees are typically paid by the buyer and can range from 3-7% of the property purchase price, plus 19% VAT.- These fees are due at closing and are part of the overall transaction costs.6.

(You said you could purchase them $20-$15,000 below market value.