We have a sufficient amount of security deposit, so I am considering charging them the remainder of it's useful life (+/- 90%) based on 20 years, purchasing a new one, and offering them the old one if they wish to haul it away at their cost.

That would definitely be an awesome price in the vast majority of markets.

Is email notification sufficient or do I need to post it on their door?

This should be sufficient to cover the expenses of the property (including vacancy and capex reserves!) 

The FHA Appraisal also has pretty specific sections including: No damage to the foundation, roof, or exterior, Safe access to the premises, Working utilities, No exposed wiring or other electrical systems, A permanent heating system that provides sufficient heat for the home, No peeling or chipping lead-based paint, Access to clean water, No termites or other wood-destroying insects, Access to, and ventilation in, attics and crawlspace, No soil contaminants, such as from a damaged underground storage container, No safety hazards, such as stairs without handrails, Compliance with local zoning regulations.

We're also wondering if we put in writing the following and they agreed via email if that's sufficient:#1 they are still financially responsible until we are able to rent the space#2 we're charging a turnover fee $1k to let them out of the lease early and collect that immediately#3 security deposit will be returned according the the RI tenant laws (ie place is left in same condition as move in)#4 room is made available at reasonable hours for showings/open houseOr do we need to hire an attorney to provide a lease addendum stating an early termination fee?

If you get into a 3 or 4 unit, you will have to perform what is called a "Self Sufficiency Test." 

I can tell you that Facebook is a great place to get leads, but the vast majority of them are crap applicants that aren't ready to rent or they're not going to qualify.

The vast majority of lawsuits against Landlords are for wrongful eviction, security deposit disputes, and Fair Housing Violations.

These financial figures will not only expose the true value of an investment property but reveal its bottom line.Also you can partner with someone who is already investing in multifamily deals as it is very vast field and requires specialized skills and team of professionals.You can start by joining local real estate clubs, attending meetups and connecting with local agents and investors.Do a thorough local market research and analysis.A local Investor-Agent can help shorten your learning curve and save you a lot of headaches as they tend to understand the market better.All the best!