Most who struggle with construction approach ground up construction using a generic cost per sq ft analysis which often backfires. 

But your subject line is pretty generic, try something that is more descriptive like a newspaper headline, you will see some good responses.

With that in mind, I’m considering purchasing four rental units over the next four years to build a stream of income that could help supplement my retirement by age 68.The challenge I’m facing is that the deals I’ve come across so far don’t make financial sense.

- The market is in a little bit of a lull right now (buyers market), its a great time to buy, but slower time to sell (my primary business is flipping houses) I can supplement my flipping business by providing project management services/consulting, helping investors find discounted properties, etc. etc.

It's an estimated cash on cash return given current rental rates subtract expenses assuming 7% interest rate, 10% management fee, 5% repairs, 5% capex and other expenses like mortgage, insurance, tax. it's a estimate to tell you what properties to analyze vs ignoreyou can see the are pockets of negative returns as well as pockets of positive return. this is to supplement the data @Devin Conley provided

You'll get better answers and less generic advice with a question like that. 

You can also check meetup.com or search Facebook for real estate investment groups, clubs, or meetings in your area.Here is my generic beginner advice.1.

Hello Guys,As someone who’s built sales funnels for marketing agencies, I’ve been diving into strategies that could work specifically for wholesalers—and I wanted to share a few ideas that might help you streamline your lead generation and close more deals.3 Proven Funnel Strategies for Wholesalers:* Capture Leads with a Targeted Landing PageSkip the generic “Contact Us” pages!

I'd be heavily focused on mitigating that future tax liabilty rather than directly supplementing that income.

They wouldn't be coming to you.So you can get a gold standard investment advisor and give up .5 to 2% of your return, or you can send the money to one or more huge index funds like Vanguard 500 index fund and sleep well at night.But if you think you have something of value other than the generic value of money, you will be sorely disappointed.