The guru your son is learning this technique from is likely not telling your son that the deal will almost certainly encourage your son to get an "off-market" deal from a "wholesaler" who is unlikely to properly disclose the details of the property or encourage an inspection. 

Hard to say if it’s a good deal without the most important piece of data, what is the fair market value? 

Any recommendations for free tools to do market research?

I suspect most who requested cash for keys had previously received cash for keys.What I do instead is explain:- the housing shortage in my market and that an eviction will forever make it difficult to find a quality rental in this market. - I will ding their credit such that all credit cards and other debt will cost them more.- I will keep their security deposit for funds owed.- I will do my best to garnish future wages until their debt to me is paid in full.This has worked 100% time so far.  

It is not an exact science, and can be subjective to the market. 

The investors I know finding properties on the MLS are making 30+ offers a week, embarassing offers, in hopes of finding 1 or 2 properties a month.Others are door knocking, cold calling, spending money on mailers to find off market properties.  

Market rent is $1400-1500 for each unit.

basically if anyone has any tips when it comes to researching plots of land in order to make the right purchase i would greatly appreciate it get with a realtor. we underwrite land at no more than 10% of after built value. in our market we build under the average new build value which is about 515k. we like to be around 420-450k and smaller footprint about 1500 sq ft with attached garages. we build close to urban core where everyone wants to live and net migrations match up. but suburban is fine. if you give more context I can help but in most markets land is a buyers market. most people aren't looking at it or buying it. 

Bought undervalued (market value of 265k); refinanced about 1 year later to contribute to next property purchase.

*I know the markets really well even better than some local agents.