If you're just looking to fund a handful of deals a year you  may be OK (consult an attorney) but definitely do your due diligence.  

I put my money where it’s going to perform best, either in real estate or my brokerage account in the form of an S&P 500 index fund.

In the case of a successful tort against you, you might regret commingling funds in a single bank account, but I can't see how keeping them consistently separated would ever burn you, even if it is a bit less convenient.  

I would like to use the home as collateral toward funding on a investment property.

I can’t believe you found a renter with a lawnmower and the desire to mow.

After it's rehabbed, you could then rent both units and use the cash flow to offset another primary residence or potentially refi and use those funds to scale your portfolio.I wish this policy change was being talked about more because it unlocks a creative new strategy for investors in MA, since you do not need to owner occupy to do it! 

The search function leaves much to be desired, but you'll do best searching "FlipSystems" as one word:https://www.biggerpockets.com/forums/79/topics/1182640-lesso...https://www.biggerpockets.com/forums/67/topics/1204806-seeki...https://www.biggerpockets.com/forums/48/topics/1098187-flips...https://www.biggerpockets.com/forums/79/topics/1188268-flips...https://www.biggerpockets.com/forums/92/topics/1181695-is-fl...I admittedly have not read every reply, but it seems like a sea of red with "Amy B." being the only positive sounding reply on this thread: https://www.biggerpockets.com/forums/67/topics/1174732-calli...Is that the person you spoke to as a reference that's his girlfriend?

These strategic improvements modernized the home, making it move-in ready and increasing its desirability among buyers.

If you can leave the property alone and use your funds I would. 

@Sharon Israel - Since you mentioned "community" as one of your key desires, I would recommend you check out Jack and Gino's community for multifamily investors.