My goal is to own more rental properties, and was wondering whats the best strategy to achieve this, with regards to what to do with monthly excess money. 

My strategy best aligns with the small and mighty real estate investors.

Very few banks will do Helocs on investment properties right now.

We toured their facility & having done a lot of rehab etc I was impressed with the construction & designs.At the time we visited they were building components for a commercial with high end units above a store front strip that was very well done.

The fnma guide is best reference there is handy chart in there https://selling-guide.fanniemae.com/sel/b3-3.1-08/rental-inc...

As a new property manager I’d like to be on top of that if someone asks me, what I suggest would look the best and be the best in their rental units.I had an awesome property manager who would buy the modern nice looking flooring in bulk so when his owners needed to rehab a unit he sold them the modern vinyl plank flooring for cheap.

@Andrew Steffens answer is the best way to handle payments from clients.

Very nice!

This will likely give you best terms available, especially over buying with a heloc.

Best of luck with your search!