Brian,One more comment as to why it so important that a person does not go into business with their retirement plan by receiving fees, salary or other compensation from any investment activities of the plan.The IRS has a division known as Abusive Tax Avoidance Transaction (ATAT) who is now targeting promoters who have people roll their IRA or 401k money into a C-corp or other entity and then go to work for the corporation.The IRS calls this a Roll Over Business (ROB) and the penalties are severe.

Following are the similarities and differences between the solo 401k and the self-directed IRA.The Self-Directed IRA and Solo 401k SimilaritiesBoth were created by congress for individuals to save for retirement;Both may be invested in alternative investments such as real estate, precious metals tax liens, promissory notes, private company shares, and stocks and mutual funds, to name a few;Both allow for Roth contributions;Both are subject to prohibited transaction rules;Both are subject to federal taxes at time of distribution;Both allow for checkbook control for placing alternative investments;Both may be invested in annuities;Both are protected from creditors;Both allow for nondeductible contributions;Both are prohibited from investing in assets listed under I.R.C. 408(m)The Self-Directed IRA and Solo 401k DifferencesIn order to open a solo 401k, self-employment, whether on a part-time or full-time basis, is required;To open a self-directed IRA, self-employment income is not required;In order to gain IRA checkbook control over the self-directed IRA funds, a limited liability company (IRA LLC)  must be utilized;The solo 401k allows for checkbook control from the onset;The solo 401k allows for personal loan known as a solo 401k loan;It is prohibited to borrow from your IRA;The Solo 401k may be invested in life insurance;The self-directed IRA may not be invested in life insurance;The solo 401k allow for high contribution amounts (for 2017, the solo 401k contribution limit is $54,000, whereas the self-directed IRA contribution limit is $5,500);The solo 401k business owner can serve as trustee of the solo 401k;The self-directed IRA participant/owner may not serve as trustee or custodian of her IRA; instead, a trust company or bank institution is required;When distributions commence from the solo 401k a mandatory 20% of federal taxes must be withheld from each distribution and submitted electronically to the IRS by the 15th of the month following the date of each distribution;Rollovers and/or transfers from IRAs or qualified plans (e.g., former employer 401k) to a solo 401k are not reported on Form 5498, but rather on Form 5500-EZ, but only if the air market value of the solo 401k exceeds $250K as of the end of the plan year (generally 12/31);When funds are rolled over or transferred from an IRA or 401k to a self-directed IRA, the amount deposited into the self-directed IRA is reported on Form 5498 by the receiving self-directed IRA custodian by May of the year following the rollover/transfer.Rollovers (provided the 60 day rollover window is satisfied) from an IRA to a Solo 401k or self-directed IRA are reported on lines 15a and 15b of Form 1040;Pre-tax IRA contributions on reported on line 32 of Form 1040;Pre-tax solo 401k contributions are reported on line 28 of Form 1040;Roth solo 401k funds are subject to RMDs;A Roth 401k may be transferred to a Roth IRA (Note that from a planning perspective, it may be advantageous to transfer Roth Solo 401k funds to a Roth IRA before turning age 70 ½ in order to escape the Roth RMD requirement applicable to Roth 401k contributions including Roth Solo 401k contributions and earnings.)

To add on to the good points made by @Ashish AcharyaIf you are self-employed with no full-time w-2 employees, you can set up a Solo 401k & rollover funds from a non-Roth IRA as a tax-free direct rollover and then invest in real estate.

He then can rollover almost any other type of retirement account into it with the exception of Roth IRA.

I have no money to roll over to the account, so I will be funding these from scratch.  

If you rollover your $100K from traditional IRA into SDIRA, your profits over the next 25 years will be tax-deferred, but not tax-free (unless you convert into Roth IRA now and pay the taxes on $100K).The Solo 401k would work the same.

Hi, I have saved enough in my retirement accounts(I have rollover IRA's from old jobs) where if I wanted buy a property in a self directed IRA I could.

If you have a rollover IRA you could try to:1) Ask for a seller carryback that is equal to the value of the down payment and the closing costs. 2) Then take the same amount of money out of your rollover Roth.  3) Use the rollover Roth Money to close the loan.4) Use the seller carry money to replenish the rollover roth in under 60 days.5) Sit back and enjoy 100% financing.

If you change companies, you should certainly be able to "roll over" your 401k funds to your new company's 401k without any penalty. 

Lastly another option is to invest your 401(k) funds in a rollover as business startup (ROBS) which entails forming a C Corporation that has been funded with 401(k) money and then you can run it is a real estate operating company.