Last night we looked at this house and it was from 1953 and NOTHING has ever been replaced except the roof, air conditioner, and water heater.

New appliances, furnace, central air, and roof.

Well, I've been living in this house for quite a year now, and the Landlord is always being disrespectful with us and always taking too much to fix things in this 10 years old house that is falling in peaces , last week our a.c broke, it wasn't getting cold, so we asked him and only 4 days later someone came to "help", and the a.c. was working again for 2 days, and then it got it broke (the air made an explosion sound and there was smoke coming out of my son's bedroom air ventilation) so we turned the a.c. off and open the windows so the burning smell would go away and then we called the Landlord and told him about what happened and that we need it him to call someone to fix it or something, anything.

The house still needs central heating and air.

The unit dial concept is OK if you don't mind having to replace those that disappear as each tenant leaves.Be aware of the thermal o/load if the fans get too hot usually because something is blocking the air circulation.

As mentioned the legislation regulating STRs in San Diego is up in the air still.

I should clarify a bit, all the homes I've looked at so far have a furnace but no central air.  

They then use a dryer that blows hot air through the bag and it expands, bursts the original pipe and hardens into a new pipe. 

Don't wait for the furnace is blowing cool air when its 5 degrees phone call-- Have a process to make sure your HVAC filters get changed at least a couple of times a year (Don't trust the tenant-- it's your furnace).

In my opinion, spray foam insulation, UV reflective paint, fresh air systems, and solar panels should be standard in all new homes built in the last 2-3 years at an affordable price.