Electrical – Open Compute Server needs 220v to run. Can I use voltage convertor to run it

Yes, you can use a 40w transformer to power your 37.5w load. However, given the quality of these small power supplies, running at 90%+ all the time will cause heat and premature failure probably.

You will need to drive the LEDs at 12v. Running them at 15-18v (or anything over 12!) is a bad idea for the life of the LED. You will need to put a resistor on an 18v supply to bring it down. I'd recommend simply getting the 150w 12v supply. Wattage/amperage is different than volts, where you can use a higher powered power supply and the connected lights will only draw what they need (example: running a 12v 50w light bulb from a 12v power supply rated at 150w is fine. Running a 6v 50w bulb on the same power supply is not fine).

Yes, you could buy and install a step-down transformer. However, heating appliances use a lot of power (an electric grill might use 900–1500 watts) and the corresponding transformer will likely be as much as the price of the appliance. Here are some which might do. Notice that a 1500 watt transformer weighs 18 pounds (8 kg)! The transformer should be rated at least twice the nominal load of the heating device to handle the cold start power curve.

If you are in an area with unstable voltage, a regulated transformer would provide some protection against under and over voltage. See these.

It is certainly better to buy an appliance which adapts to different voltages—which is easy with most electronic gizmos like computers and cell phone chargers—or is made specifically for 240 V.