My question is; should I save all receipts of purchases for the house (vanity, toilet, dishwasher, flooring , materials for adding value to the house, etc) or is that overkill?

If yes, then you're going to need at least 1,001 hours in a real estate trade or business in which you materially participate.1,001 hours might be a stretch for two units and would raise some eyebrows. 

Figuring out a job and material/labor is a no brainer now.It's usually what's behind the wall or finding unseen problems that has me now always adding 20%.

Here's how you can calculate the asking price for the land:Estimate the value of the newly constructed home: This can be done by looking at comparable properties in the area and considering factors such as size, features, and market demand.Calculate the development cost: This will include expenses such as materials, labor, and any necessary permits or fees.Subtract the development cost from the estimated value of the home: This will give you the land value, which can then be used to determine the asking price for the land.For example:Estimated value of the newly constructed home: $500,000Development cost: $100,000Asking price for the land: $500,000 - $100,000 = $400,000I hope this helps!

I don't have an intense process driven off market acquisition strategy, but I'm happy to share anything that I have learned along the way. 

HOA/POA - If there is an HOA, be aware of what the dues are and what their requirements are for what you build (ex. square footage requirements, construction materials, etc)4.

Do I need to have the type of materials I want used?

Some investors have had luck going to home depot in the morning and poaching the contractors as they show up to buy materials for the jobs that day.

It used to be my favorite loan program.Duplex w/ FHA 3.5% down is a California relevant sweet spot.3-4 unit generally do not pass self-sufficiency due to rent/price ratios that are driven by appreciation, so it's 20% down for that. 

The marketing materials to sell these plans really oversell the benefit.