Precious metals and coins were a much safer bet years ago, not sure I'd invest in them right now.  

Given the current uncertainties, I would probably buy precious metals, evenly split between gold and silver and wait for the next correction.

Are metal doors are better or wooden doors?

I decided to add a few more items to check on.Climate controlled- humidity controlled.Temperature controlled- heating and air.Prior to building check on floor insulation, Ceiling metal to metal insulations contact points- to reduce heat transfer from the roof to the building frame inside, and also try to do Spray foam insulation to get a great seal.If your building is up already.

To give you an idea of the requirements, at least half of the Corporation’s assets would need to be invested in real estate that is directly managed or developed by the Corporation.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); andNeither may be invested in your own business. 

The best thing I can say is if you’re not comfortable buying individual stocks, look into ETFs that track the index, specific sectors (like financials, tech, etc), metals, and fixed income (bonds, treasuries, etc).

It appears that the receptacles in the main living rooms have no metal box and are not fully grounded.

On the other hand, if you are looking to invest in alternative investments then a solo 401k or an IRA LLC may be a good fit.Following are the similarities and differences between the solo 401k and the self-directed IRA.The Self-Directed IRA and Solo 401k Similarities Both 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); andNeither may be invested in your own business.  

To qualify for a solo 401k, you must be performing at minimum part-time self-employment and have no full-time W-2 employees.Following are the similarities and differences between the solo 401k and the self-directed IRA.The Self-Directed IRA and Solo 401k Similarities Both 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); andNeither may be invested in your own business. 

@Peter SchuylerThe following IRS web page is a good resource regarding what constitutes self-employment. https://www.irs.gov/businesses/small-businesses-self-employed/self-employed-individuals-tax-centerFollowing 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); andNeither may be invested in your own business.