I agree with @Jonathan Greene that the question is vague, so my advice will have to be as well.The short answer: MTRs are real estate, so just like all real estate, any place that can long-term-rent cash flow with increasing jobs/population can be a suitable site to purchase. 

Natural appreciation usually takes holding the property for at least a few years (just depending on what the RE market does going forward).  

In any case, you need to make sure that the HOA will let you use it as a short term rental.

I would rent 3 or 4 on long-term to get someone in and paying right away.

I'd underwrite the property, with that low interest rate assumable mortgage, to cash flow positively from day 1 as a long-term rental, but be willing and able to use the rent by the room strategy or to rent out part of the property as a Short Term Rental to dramatically increase cash flow during my occupancy. 

Most of the bad noise right now is about downtown skyscrapers and REIT or Syndication apartment complexes where they played short term versus long term interest rates wrong.  

For example I know many who are now just lending because they are getting 10-12% short term which is far better than any cap rate on real estate. you do not have the equity upside as you do real estate but its a way to play the short game to see what happens over the next 12-24 months and decide when you want to buy.

Since you’re considering converting the upper unit into a short-term rental, you may also want to implement strict rules around smoking or strong odors in your house rules to avoid future issues.

There aren't many "short-term, get rich quick" type investment options in SoCal today. 

The only time I'd say it's an okay idea is if either: The area you're investing in is expecting so much appreciation that you will easily profit off that (watch out for mortgage interest though...the appreciation needs to be higher than what you pay in that and expenses to make it profitable)You have some specific reason for wanting to do it- family ties, future home, major long-term hold, etc.Maybe I'm forgetting another one, but those two are all I have for now...