Build time can be calculated from the Cost. For buildings that must be purchased with coins, divide the cost by 62.5 and round down to get the build time in minutes. Ex: Marine Turbine 15,200 / 62.5 = 243.2 ==> 4h 3m
Deciding the best return on your investment depends a bit on how often you collect. If you are collecting once a day (or less), the Underwater Cave Ride has the best Cost/Earnings ratio (it pays for itself in 15.5 collections. However, since it takes 24 hours to reach full earnings, the Infirmary is a better choice if you routinely collect more often.
The Marine Turbine and The Tethys have the best Earnings/hr. The Repair Bay is almost as good and has a very good Cost/Earnings ratio.