Should I just talk to real estate agents and contractors to see how the market fluctuates and potential problems with rehabs go?

Actually, the lender told me 3.62% but told me to use 3.75% to be safe since it fluctuates.

Here's a guiding post for you: https://www.biggerpockets.com/member-blogs/10850/81214-what-goes-up-must-come-down-or-market-cycles-fluctuation

I have seen this fluctuate wildly over time and agree with the observations on length of stay and guest age and quality. 

Second way is to assign a flat water and sewer fee to each unit that get updated annually based on actual consumption, this can be easier for the tenants since is does not fluctuate but it doesn't discourage irresponsible water consumption and the third is to actually submeter. 

In the case of a normal listing where it’s based on percentages, yes, our commissions fluctuate.

I personally don't invest for appreciation as it makes you more susceptible to market fluctuations whereas, if you invest for strong cash flow (b,c,d area properties) even if the value of the property goes down during a market correction, you're still making money.

The seller should get a yield that compensates them for the added duration of the note and thus the greater sensitivity to interest rate fluctuations.

If you have an interest in taking advantage of these common fluctuations then open up a Forex Trading account and trade the spread over longer periods of time (ie. 3/6/12/18/24 months)- not daily.

If you take out the easy money created bubble of the last decade and short term fluctuations the ONLY change in home prices since 1945 has been due to inflation.