If so, at what entry level is suitable.

I currently use the two family cars we have to conduct these activities, and I am looking to buy a new vehicle that is more suitable (a truck) for the tasks that I have been doing. 

It could offer some benefits if designed properly to avoid a "slumlord" style property with 3x units crammed on one lot.

I work with a lot of MTR investors and what I am seeing frequently with multifamily acquisitions, particularly with new investors, is that they often test the property just like you're mentioning, as a way to dip their toes in the water and see what works in their area and for their personal style.

I think the A frame idea works great for this setting, but some have said the A frame style steeper peaked roof works better. https://www.instagram.com/redtail_cabins_adu/

Location. 25 minutes to beach, 40 minutes to Florence, 15 minutes to Pisa, 60 miles to snow skiing.  10 minutes to Home Depot style store, etc.  

If I had a large amount of liquid capital, do you think I should use it to purchase fixed assets or for some project development, which would be more suitable?

It might be that this couple that owns the house is just getting their feet wet with having someone else managing the property and they want to get comfortable with your management style as being what they want.

We still end up doing a few things like upgrading appliances or installing recessed LED lighting and dimmer switches and other life-style upgrades to attract top-notch tenants. 

I really like the investment style you brought up, but c'mon Curt, if you're going to invest you should use your HELOC to buy the Yankees or Disney World!