No more/No Less, but they have the necessary custodial powers to allow me to use my qualified funds.

And on top of that (and to make a long story short) my ex-wife stole more than $800k from me when we split and we are in a nasty child custody case which costs a crap load of money.

Some people like the custodial style and some people like the speed and ease of checkbook control.   

Go to the source directly yourself and see what the rules are:https://www.irs.gov/retirement...If you are using custodial self-directed IRA you would not be able to sign the contract, IRA custodian would have to do that.

When deciding, obviously check online reviews and also understand the difference between custodial solo 401k and checkbook solo 401k. 

He says he'll be living in the house alone, but his kid may join him at some point during the lease (he's trying to regain custody). 

Then there's the remaining assets in each of our custodial accounts - looks like we'd also be paying annual fees to both that custodian and the new LLC custodian..

Have you considered a checkbook IRA instead of custodial? 

You'll also have custodial fees with them. 

@Waylon GatesThere are many reasons the Solo 401k is the preferred structure.Contributions limits are roughly 10x higher, there is no custodial requirement for the 401k, you can take participant loans, you don't need the additional expense and administration of an LLC to have checkbook control, there is a built in-Roth component, a spouse can participate, there are additional tax benefits, there is generally greater privacy, and the plans are often quicker to setup and cost less money over time especially compared to most IRA LLCs.