It is very well built but what really piqued my interest was the potential to convert the formal dining to a 4th bedroom simply by adding a door. 

Finding buyers is the easy part, securing the contract is what you need to be focused on.If you have good deals, you’ll have investors banging down your door all day long.

This will work really well with larger multifamily, but it can be beneficial at any level. 

... to take.Have a door in a run down condo in a good urban location that got an offer by an investor that now has overwheling majority of building owned.

My house is a 2 story (3/2) upstairs and downstairs 1/.5 + a huge family room and a bunch of closets I will never use.bought in 2009 for 745000 in the SF Bay Area Peninsula4/2.5 everything re-done in 2008zillow/trulia shows $1.4MThere is 1 exit through the garage and 1 private entrance through a side door/side yardI need to install a small kitchen , a shower and move the laundry room into the garage.

It probably does not really "save" on taxes in a tax-deferred structure, but would allow you through the benefits of tax-deferral to mushroom that income to a larger amount over time. 

Things progress from there to scaling with larger multi-family properties, commercial real estate, and so on.I'm wondering what experienced investors would do with the knowledge they've gained if they were starting from scratch and had $500k-$1m starting capital.

More doors the better

The difficulty that I am having is when it comes to larger items, such as New HVAC and knowing how much to put away given its useful life when a property is acquired, Painting a property, new roofs (along with determining its useful life), redo of blacktop for parking lots (especially older properties).....these larger capital expenditure items are a lot more difficult to determine in my opinion aside from putting money away for unforeseen maintenance items or vacancy. 

If it can bring close to $100 dollars a door and at least 10% cash on cash return.