Purchase price : $80KMajor Rehab: $50 k ARV: 170K-175KRent : $1600/monthRehab: Based on quotes I received from 3-4 contractors: new roof, foundation fix, sheet rock, new flooring, landscaping, new bathroom, kitchen, siding.

I asked a few questions such as; Is there any known foundation issues?

With the dry ground and soil conditions, foundations tend to move quite a bit.

@Jay Santos While this may be more of a "back to basics" foundation book, I would also recommend Rich Dad Poor Dad by Robert Kiyosaki (Although you may have already read it, given that you are here on BP). 

I am planning on building a ADU in my backyard and I was wondering if I should just hire a general contractor and pay lot extra or do my own project management  by hiring individual contractors/sub-constractors to do the job.Like - Arhictecture and permit person, plumber for all my pluming including addition sewage and water lines , foundation and framing contractor.

Definitely get your personal financial situation stable so you have a good foundation to leverage off of for years to come.

Look at areas that rents are the highest but houses can be found at a discount (foreclusures, shortsale, for sale by owner.)Once I kick my RE biz into high gear I plan to find off market deals through direct marketing.

Can someone please let me know roughly  how much it will cost me to add the foundation. 

I bought it cash because it didn’t have foundation so I had a great price.

It breaks down the process step by step and provides a good foundation to learn from.