@Yinna Wang - Wholesalers can definitely find off-market opportunities, but their interests are not always the same as yours and therefore, the property value to them and you may be much different. They are looking for a quick trade to make money to offload early so it may not be what you are looking for.
Any numbers that get tossed around on the podcasts in terms of door/pct are location specific. With taxes as high as they are in NJ, the numbers are different. Owner/occupy is a great first option, but you have to be prepared to be an on-site landlord. So, unless you are buying in an LLC, which is preferable, the tenant will know their landlord lives next door. This means 1 am knocks on your door because their toilet doesn't work.
Block value can be assessed by looking at an overview of all the median values of homes on the block to assess where your deal fits it. You will want it to be at the low end so there is high return with time. Also, multi-family homes in Northern NJ are often on blocks mixed with single-family homes so you need to be able evaluate the difference in value between them.
You only need a contractor to give you reno estimates and you definitely want to find one of those on referral from someone you trust. I hope this helps.