Have your agent manifest sold comparables within the past 90 days to give you a sense of what the market is taking.

If there are limited comparable properties, especially direct comparable properties, this makes appraising a property more uncertain and thus places greater liability on the appraiser.  

You can then compare that to the company you mentioned and pick the best fit for you.The primary item to watch for would be prohibited transactions.

I would not personally compare it to the stock market because (among many other reasons) it is difficult to get to know the CEOs or the companies that you are investing in the stock market.

I think there's always a good reason for why the rents are lower than market including: older building, older unit interiors, the specific neighborhood or the lack of amenities in that specific property compared to other 'market rents'.

All:  How would you compare the relative security (of your investment) for a warehouse/logistics center with office space with a national tenant like OfficeMax, Staples, or similar with a NNN lease with several years left and renew options; vs. a large 100-200 unit multi-family with decent occupancy in a good area?

This is lower rate, but yes you pay the interest right away compared to HELOC when you just pay when you pull out the money.

According to the MLS 19023 home prices were up 27.4% in August of 2021 compared to last year, selling for a median price of $131K.

2-4 unit properties are convoluted with more than one valuation method being both the income and comparable sales approach.

Definitely not the case when being compared to the better investment opportunities.The OPs ADU will be initially cash negative.