As soon as the house is built, I want to start a new build on this lot to sell as a spec house, budgeting approx $450k build cost, sale price is estimated $750k, supported by comps.

The biggest mistake I see is to worry too much about the cost of things like lawn mowing and water bill and the exact percentage for vacancies, but not being enough aware of the general condition of the property - resulting in 20k, 50k or more in capex a few years down the road.My advice: look for a quality property that has all the big-ticket items already done.

Rehab project was supposed to cost $50k.

This isnt true -- but with anything, convenience costs. 

Once the problem is resolved, find a qualified PM company and transfer the management.A lawyer is going to cost drastically more than $28 per month.

See here a couple diagrams to help you in this quest:Asset Protection Decision Diagram - to help assess the need for asset protection, and what to implement : https://www.biggerpockets.com/files/user/CosIorg/file/asset-protection-decision-diagramAsset Protection Onion Diagram - what, when and at what cost one should implement in terms of asset protection - https://www.biggerpockets.com/files/user/CosIorg/file/asset-protection-onion-diagram-v2

So they take a 4 bedroom house and make it a 6 or 7 bedroom, which will cost you money to do initially.  

While initially enthusiastic about the potential of the property, upon inspection, I found myself overwhelmed by the extent of work required for its rehabilitation.As a beginner in this field, I must admit that I feel ill-equipped to accurately assess the necessary renovations and calculate the total cost of rehabilitation.

It's key to keep learning, look into different ways to pay for places, and think about ways to share housing costs.

The plumber who I spoke to says the pit in the picture is really big and the cost of labor plus gravel/concrete to make the pit smaller plus the cost of replacing the ejector pump is 5K.