Heatsink design, factors to consider?
Does vertical heatsinks have any advantages over horizontal heatsinks?


For this small scale and low temperature delta, and with forced airflow, there will be no appreciable cooling advantage. Convection will be dwarfed by the airflow from even the weakest of fans.


  How can I calculate the cooling capability of this heatsink?


"There's an app for that." Seriously, there are many good calculators online. Find one that you like. Many of them link to articles explaining how the calculations are done, in case you want to go down the rabbit hole. Here's a search link. 

https://www.google.com/search?q=heatsink+calculator


  That regulator which is going to dissipate more heat, should be at the bottom side of heatsink or top?


I would place the hottest component near the middle where there's more aluminium to spread the heat in all directions. And perhaps place the less hot components some distance away from it. It will of course have to be a compromise.


  Where should I install the temperature sensor to monitor the temperature and change the fan speed?


I would keep the sensor out of the airflow, but close to the hottest component, with good thermal contact to the heatsink.