If you need to borrow (get a HELOC) to make the down payment on your first property, it's likely that you are undercapitalized. I'm firmly in the camp that believes you should be able to manage the down payment and initial repairs (plus an ample margin) in cash. Better to save longer than to get yourself overleveraged and unable to properly manage the property.
As to how much you should be saving for upgrades to the property, that's hugely dependent on the kinds of property you'll be purchasing, and on the individual properties themselves. If you're purchasing brand new properties, your upgrade budget might be near-zero. If you're buying hundred-year-old buildings, it might be significant. Or you might be buying a hundred-year-old property that has been recently upgraded. Or you might be buying a hundred-year-old property that doesn't merit being upgraded. There's just no way for us to say.