My understanding is that they typically call up and go to the section 8 office directly to get a good relationship with those working there, who can then cherry pick the best section 8 renters for them.

As investors we don't pick guarantees, we assign probabilities, allocate on conviction and then manage downside risk where possible. 

We have an opportunity to pick up a lot and are wondering if it's cheaper to just buy a duplex/4plex that is already built and fix it up instead of building one from scratch with the current  construction costs being so high. 

@Angie Guifarro I bought investment properties first and now house hack a duplex I picked up along the way.

Pick a driving radius.

Try picking on the low hanging fruit first, "Pre-foreclosures" you already know they are in trouble.)Would also “drive for dollars” by biking and running but only in my local neighborhood here in Northeast LA.

All this time and energy spent vs picking up the phone, calling tenant and saying "Oh hey, curious where all this is coming from, kinda caught me by surprise".     

Pick a primary exit strategy that you like. 

Or can you pick up some additional contiguous land for cheap, and put your retention basin on that land?

If you have inventory in your area that require renovations, distressed homes, outdated and you can pick it up for a good deal.