I'll second @Jeff Copeland,
Buildium is very scalable. We started with 20 or so units and had the same priorities. We have since grown from there and handle pretty much everything directly from Buildium (I often check it before I check my email inbox in the morning). Certainly not cheap, but our favourite option so far. It will also offer structure based on best practices for management so that can help you get more organized and productive.
Stessa is also great, especially if you're mostly focused on accounting and tracking rents.
Avail is newer and free. I've not used it, but free isn't a bad price.
- I did use Stessa briefly, but ended up just switching over to Buildium pretty quick; if you're jumping onto a moving train (exisitng investments and business model) then paying a little for software isn't a terrible idea. Other folks on here will tell you not to bother, but many don't understand the "nuance" of taking over family properties.
-- You're also going to be told you should just hire a reliable Property Manager. It's good advice, but again, if you're seeking out some structure already I count that as an indicator that you're off to a good start. Just don't be afraid to ask for help or admit you don't know things (in our market, we offer a few a la carte services for that exact reason..no one knows everything). If you're ready for the work (and ready to learn) and you feel like you and your family can set appropriate boundaries and expectations then we're behind you 100% and happy to offer advice. Just make sure you know you're signing up for a lot more than simply staying organized.