Dude, you're only making $300/mo without including anything for maintenance, vacancy, management, etc?
I would carefully reassess your plan to buy more of this type of rental. At the very low end (where you're at) the overhead can just kill you. One $10k new roof (which will definitely happen, they only last 20-25 years) will wipe out years of profits on a low end property. You'll need paint and carpet regularly, you'll need appliances, the sewer line will crack, etc, etc. None of that stuff costs much more on a $3k/month house than an $800/mo house, though.
Any freestanding SFH needs minimum $1500-2000 a year to maintain it, unless it's cinderblocks with a metal roof or something.
Assuming you want to keep doing this, you need to decide if you want to go the max leverage route (riskier, but more profitable most of the time) or pay off mortgages as you go. That's more of a philosophical decision than a financial one, though, honestly. You can lose all your properties with leverage - or get rich fast. If you pay cash, you're not getting foreclosed on but you're not leveraging that, erm, leverage and cheap gov't subsidized money.
-W