I don't know how they will all hold up in a crash like 2008 though?

Every 30 years or so there is a crash in housing. 

People have been talking about a "crash" for years now, and wise folks who bought anyway have now enjoyed that many years of outstanding appreciation in Denver, while tenants are paying down their loan principals. 

People will tell you the market is too slow, too hot, about to crash, or too saturated.

If a major housing crash happens, I’ll survive because my cash flow numbers are so high, and I could survive for several years even if none of my properties were rented.

Consider yourself extremely lucky.If you want a crash course in wholesaling, which is sounds like you need, there's a short book that outlines the whole process. 

Is your advice to wait for a market crash/prices to fall down or is there another area (in RTP) I could look?

if the housing actually crash's like 08 your Heloc will be frozen just like 08 when almost all helocs were called or frozen.you would need to evergreen it before the crash.

I’m a big fan of the Brrrr strategy and am studying to use it, but if after I refinance a property for 75% and am left with 25% equity, is that enough cushion for a big market crash?

During that time I bought four rental units before the market crashed.