@Steven Cowles All the things mentioned above as well as checking out radius.com they have some good software for upcoming development/expansions.

Also, any online software that anybody would suggest for rent payments?

This includes a good maintenance staff, PM software, and great customer service. 

Our credit and background checks are all done through our Property Management software Buildium but Smart Move with transunion is a good service for smaller landlords. 

You can also buy lists from companies like list source and there are some REI software companies that have a cash buyer search feature.Best way to find motivated sellers is to purchase lists and compile lists from city and county for 60-90-120 days late on mortgage, pre-foreclosure, delinquent taxes, water cutoff, code violation, probate, divorce, high equity, etc. and mail or cold call.Other methods are driving for dollars, door knocking, wholesalers, craigslist, Realtors, FSBO sites, expired listings, foreclosure sites, put out bandit signs, billboards, newspapers, other offline publications.You can use pay per click/adwords, facebook, Linkedin, Youtube, and other forms of online and social media.

An app is an inferior software program designed to work on cellphones and tablets.

Sure.The easy part is picking a landlord software to use, that'll help with a lot of tasks. 

Short into: I'm a software engineer looking to branch out—currently in the research phase of things.

Once you do decide to procure your first rental, definitely lean on the BiggerPockets community as a resource; I'll also say that it could be worth considering a shot at a software solution built for DIY landlords (1 - 15 units) to bring some automation to the rental management process (just to make things much much easier for you)!

Is there any software you'd recommend over just basic Quicken?