He brought 100% of the down payment of 60K and we jointly took out a loan for the remaining 200K.

I plan on starting in September as well as continue working counter terrorism in the metropolitan area, a joint task force.

I have brought it up with people and there is genuine interest, however I am not sure how to articulate possible terms of the agreement, think elevator pitch. 

You can do a joint venture as @Christopher Lombardi had mentioned.  

The better solution is to have two separate IRA LLC entities.The two IRA accounts may form a LLC jointly, so long as they both participate in the initial inception.The LLC will not, however, be able to accept future new contributions from either IRA. 

I'd like to develop a long term working relationship with them to file joint taxes with my spouse - we both have multiple taxable income streams.  

To answer your question, the initial interest to partner together came from the joint interest to purchase the same piece of property.

We sometimes forget that there are genuine people out there and have a common agenda/purpose all a while doing amazing things.

Worst case you have someone who is genuinely trying to screw you (but I doubt that if they have not received the funds yet).

@Morgan Cooley If you are just getting started, how about you find a deal provider with some experience and form a partnership or joint venture agreement.