I’ve been fortunate to be given stock options back in 2012, and contributed in the ESPP program offered through my company.

Am I allowed to contribute to his solo 401K as his working spouse?

I want to invest deeply and and learn more these conversations are so profound and elaborate, thanks everyone who has contributed I don't understand all but am learning from these forums muchas gracias mi gente.

It would be easier and simpler if both contributed equally and jointly, such as financial and work.

Unless you are contributing funds to your plan from self-employment earnings, you cannot take an asset you own currently and put it into your Solo 401k.

Seller contribution. 

@Kristopher Devers You need to know before making an offer....to determine the probability you may need to contribute $2-$10k in addition to the purchase price.  

Would the amount you paid in closing costs contribute in any way to your equity in the property?

I would contribute all or a portion of the down-payment and my name would be on the deed.

For those familiar with the area, are there other factors that could be contributing to vacancies much of the year e.g outside of incompetent property manager?