I just set up my solo 401k under uDirectIRA, and the LLC that will be 100% owned by the 401k, and ready to buy real estate using the LLC.My question is regarding taxation. 

@Selina GiarlaI recommend finding an accountant who specializes in real estate taxation, business taxation, tax planning and financial planning.You may want to consider working with your accountant remotely to expand your options.I would also recommend looking for a accountant willing to work with you throughout the year.

.- Purchase price: $250k- Sale price: $350k- Depreciation: $9k => Roughly ( ($250k*.50)/27.5yrs) *2yrs)- 2023 marginal bracket: 32%What is the proper taxation?

I also recommend hiring an accountant that specializes real estate taxation. 

When you sell your rental property in the future, you can roll your capital gains into another investment and not have to pay tax at that time.

I am an Enrolled Agent licensed by the IRS and specialize in real estate taxation.  

@Rob CucuglielloI recommend finding an accountant who specializes in real estate taxation, tax planning and financial planning.

@Erwin Groenendijk, you are correct but Americans should be able to credit their taxes paid in Spain against their equivalent US taxes courtesy of the treaty for the avoidance of double taxation between the two countries.

I just set up my solo 401k, and the LLC that will be 100% owned by the 401k, and ready to buy real estate using the LLC.My question is regarding taxation.

As always, the most detailed and complete answer from "The Black Belt in Real Estate Taxation".That's exactly how I imagined it worked.