Within the city limits, I believe STR are prohibited, so start with understanding the regulatory environment. 

Your either going to beat your head on the wall because of the debt and stress about it making money or stress that you paid all or a substantial amount of your cash when you go all in. i understand this will be unpopular for a lot of BP viewpoints but it worked for us, we own 2 STR that we owe nothing on now and purchased both for 670k and have a real value of $1.2-1.3mm. it requires you to put a lot of your cash back to your reserves but you have it all to do it with.Personally we believe in ourselves and our ability to pick high producing properties(that is the very first and foremost consideration) then we went all in and never looked back, 2 years later bought our second STR with mostly cash. 

Or would it be preferable (and cheaper and faster) to just hire a contractor to renovate the old mobile home to HUD standards?

I understand their kids have special needs, but that isn't your responsibility.

My current understanding is that ADUs may offer better cash flow, although they may not necessarily provide superior appreciation compared to condo investments, considering the anticipated decline in interest rates over the next couple of years.Thank you for your expertise and guidance!

Verifying last 2 years of rental history and income extremely important to find the “best of the worst”.Make sure you understand the Class of properties you are looking at and the corresponding results to expect.What else can we assist you with?

But you also really need to understand the market and how to operate in it.

Zillow says I can get $2900 on a standard 1 year unfurnished lease. 

I'd also suggest running some for-rent ads, to see what type of response you get.It's notoriously difficult to accurately predict ARV on these types of properties, because they're so niche, and the market for them is usually completely different than the market for more standard properties...For instance, a lot of lenders won't provide a mortgage on a property for less than $100k--and if that's the case in your area, that would mean that the only potential buyers for the property are cash buyers, which makes selling the property more difficult.  

I can help you understand the market.