Hey Martin,
I too use QB Desktop Pro. The main advantage is that I can create multiple companies and the cost is the same. Then there is the fact that my CPA prefers QB.
For maintaining my notes, I use a CRM called Pipedrive. I've put out a few videos on YouTube on how I use the system to manage my notes. I've got a call with Salesforce today, so maybe I'll be switching to their new product. Or not.
I use DropBox for my file storage. I keep my QB files there, so I don't have to worry about losing my books if my computer tanks.
I use MailChimp for marketing, but there are other mail systems out there.
Foxit Phantom PDF for my PDF editing and signing.
For maintaining my mileage, I use TripLog. My preference for TripLog is because it is cloud based, not phone based, so if I lose my phone, I still have my miles recorded and can access them from anywhere online. The systems that only record the data on the phone can be great, but when the phone dies, so does your data.
In fact, everything I do is cloud based so that I can access anything from any computer. I actually use 3 computers for this work, but all of the computers are set up similarly and I can function on any them at any time, except maybe to let DropBox catch up on the latest files.
That's about it.