Nau,  which building/unit size did your client in Waikiki purchase?  

Since you have had no vacancy for 10 years the cost is negligible to renovate.Based on the size of your unit and the upgrades you listed warranting $875 in rent I would target pushing your rent to the top of the market. 

I spent alot of had earnt cash on somebody elses property that added value to. i didnt have any plans for the future but i knew id find something that would create the freedom i wanted. im 37 now and want to get into the real estate world.Ive been away from australia for 10 years and havent been able to follow the market trends but i know the interest rates are below 4% but house prices have increased significantly.For example i bought my land for 90000 and built on for 120000 now a block of land the same size is around 300000 and the median price for a house in my area is 420000.So i have equity that i could re invest for.

As in $200-$300/mo/unit ... this will be more accurate generally, but still vary depending on the size of the unit, number of bathrooms, fit and finish, class of the neighborhood, etc.

Now there are pockets of the historical subdivisions that jump in price depending on size you could potentially pick one up for 250-300k and resell for 450-600k.In my experience a lot of my investors like to stay in what they call the " San Antonio's sweet spot" 150-250k ARV range.

Find similar square footage homes, similar land size, and same bed / bath count no more than a mile from your subject property.

Some of the scenarios are moving to a different city, changes in family size along with some others 

This will depend on the size of the unit and then look at what property taxes (enter info here) then guess vacancy, utilities, maintenance, property management etc...Or you can use the 50% rule of thumb and estimate that 50% of the rent will be used for expenses.

I recently had a plumber come out to fix a broken sewer pipe under the foundation. After the house flooded a second time with raw sewage the second plumbing company found mud in the pipe the first plumber did not clea...

Depends on the size.