Including an umbrella policy, taxes and insurance take up 25% of my monthly cash flow.

Insurance is going to be your main issue, as it is costly and having been on three boards, we were risk averse to changing it or losing it as I recall insurers bringing up details from bbq to pets, etc. and when you have a claim, really it seemed a struggle, and if they can get out of paying they will..even got dropped once...Maybe have the master policy professionally examined to see what is allowed and, worst case, explore if you can get legal help to work out a mechanism to hold the association harmless (be it your own policy, plus umbrella, and maybe indemnify the association for any claim)....some great advice here and some firms may be more willing to write the policy.

Or should this be under an umbrella or just make a Corp?

@Catherine Brennan I would suggest getting an umbrella policy for the short time you have the property in your own name.  

On the other hand, what about the apartment buildings that don’t fall under the umbrella of new development?

If it's just a single family home you could look at getting a million dollar umbrella policy to cover any issues. 

if you're looking at small 1-4 units and you're going to be buying under 10, you may want to consider just getting a good umbrella policy specifically for landlords so you can stick with residential financing.

My question is whether (or at what point) should I consider having umbrella insurance?  

If you're buying 1-4 units properties and doing residential loans, get an umbrella policy. 

@Tom Kairys, one thing newbies miss is that it is much harder to get financing in the name of your entity vs. your personal name.You can get an umbrella liability policy and buy in your personal name, or buy in your personal name and then transfer to your LLC (risk the due on sale clause).All of these options have been extensively discussed on the forums and accessible via a few searches.