While they are touring, inquire about what businesses they plan on running out of the property.

My plan is getting Fixer-UpperWhat do you suggest for me?

Purchase like so, use 401k loan to rehab, then rent and refinance is the plan that I am considering for my first property.

Are you planning on taking out a loan for other property purchases?

I do not want spend my W2 income to cover house mortgages, to not have pressure if things like layoffs will happen, so my plan is to buy house in a good neighborhood that may have appreciation and some minimal cash flow after all expenses (management, maintenance and vacation).

Unnecessary costs for a cash-out or HELOC doesn't make sense unless you plan to live in the house for a few years.  

I wouldn't even bother asking for comps or how much they think the repairs are, it doesn't matter what a wholesaler comes up with, a real buyer will know how to assess on their own how much the repairs will be and thus how much they can pay for the house depending on their business plan (either to fix up and rent out, or fix up and flip/sell). 

The plan is that I would refinance after the property rehab to pay off the HELOC and the rehab costs, then probably just keep it as a DSCR loan or refi into a 20% down conventional.

This year we plan on doing several spec houses assuming we can get financing lined up.

Conventional route would work for your 4-unit plan, you on't have to worry about a self-sufficency test going that route like you would with FHA.