They collect the rent, deduct their fee and any other expenses they are allowed/required to pay on your behalf, then they distribute the remaining funds to you.It's critical that you review the property management agreement and understand what you agreed to.

You have the right things to analyze here, but STR might be a tall order to do arbitrage unless you have the funds to go heavy on the furnishings.

Just curious, how did you get a 9-unit under contract without a pre-approval or proof of funds?

That's what crowd funding would do.

If you’re running a MTR more like a STR, then you can decide to charge the full boat as agreed or, if you desire (possibly to obtain a good review) find a substitute renter for the period in question and credit the original renter.If, like me you operate a MTR more like a LTR just for a shorter period of time, you probably use a specific rental form provided by either the state real estate commission of the local NAR affiliate.  

Invest the $550,000 in a mutual fund such as S&P 500 or VTSAX with a 10-year history average annual return of 12% 2.

You may have to sell the whole property to access the funds in the IRA or 401k when you retire. 

Quote from @Luis Maza: Hi all, returning to the game this year and would like to pick your brain a bit, we started working with a wholesaler that will send us properties here and there with the "wholesale" price, but every time we check the properties and add the numbers, the margins are, well, I am not sure, too low perhaps, maybe we are getting to picky, but this is my dilemma for example...Today we got a property, wholesale price 300K, rehab about 50-60K, ARV is about 440k, but with the cost of hard money, we have about 100k cash, hard money interest at 11%, keeping the property for 90 days while we finish the rehab, closing costs,  commissions, we ended with 30k profit before taxes, while it sounds appealing, adding up all expenses and cost we ended up expending 50K on rehab(labor/materials) and about 50-60K with cost of the loan, interest, commissions, etc, everyone makes money, happy with that, but it seems that we are working to produce a 100k for everyone else, while we make 20-30k if everything goes well...In my opinion, we are not really getting "wholesale" price, it seems that we either buying to expensive, rehabs are much more than we would like to spend and the cost of borrowing that money is too high...We came up with thoughts as: lets fund it ourselves with money from a close friend/partner that we will bring in, and that would save us 10-15k here and there, but still, is that the norm now?

Make sure the lender gives you what is called "Non-Dutch Interest" on the rehab portion, that means you do not pay interest on the rehab funds until you use them.

With that price range you are looking in I'd explore doing just 5% down conventional for a mutlifamily rather than 20% if you want to be able to save your funds to be able to purchase again sooner.