What brand of self sealed fire/carbon monoxide detector are people buying for their Baltimore rentals.

@Ben Granja, the above advice that a bank will lend 80% of lower of appraised value or purchase price is incomplete. 

FHA does not mean the value will come in any lower.

Could this be negotiated with a lender and would they be open to accepting a lower down payment?

Their rent to "all in" ratio is much lower than what I want, but somehow they use this strategy to get up to 100 houses.

You probably aren't going to find any markets in Maryland with a lower price-of-entry then Baltimore, so if "low budget appeal" is your driving factor then focus on Baltimore and maybe Dundalk.  

However, typically if you buy cash, that percentage tends to be lower than if you borrow.

A hard money lender or private lender may have different down payment options but don't expect anything much lower than 20%.

If anything it would help lower your DTI. 

That would be an another red flag.As many realtors on BP have stated, too many folks run towards a lower rate, not realizing that a lower rate is not the end all, be all of getting a loan to closing.