The fact that you are sedentary does not matter. Normal value depends on age, male/female and body length.
Further the bicycle test protocol is of importance. Often used protocol is start at 50W or 100W and 20W/min increase.
You did not give your age but for male 187cm and age 57, normal wattage is 200Watt.
First, using USA measures 230 pounds at 6' 2" doesn't strike me as "quite" overweight.
Stress tests are designed to get the subject's heart rate up to it maximum.. taking into consideration the physical fitness/condition. Thus the METS... would depend on how much it takes to reach the desired heart rate. It is even possible to do a stress test using only chemicals to raise the HR... no exercise at all. This is used for very old people and people unable to do physical exercise: tread mill (most usual in my experience) or other.. suppose a bike could work, never seen it used.