To state the obvious you need a take-off alternate if, at time of the departure, the weather is below the applicable landing minimum.
Since you are probably sitting in the crewroom reading the weather information i would say you can disregard the metar. However, if your depature is within one hour of the issue of the metar you may use the trend of the metar. But since the trend is an improvement it must be disregarded. Hence, for your planning you must assume 100 meter vis = below even CAT3A (200 meter required) = takeoff alternate required.
In this case it make no difference, but if the metar had better values, it would be an advantage to use the trend (provided your take off is within one hour of issue).
If you take-off later than one hour after the issue of the metar, you must use the TAF. In this case you have 100 meter vis untill 7 o'clock, since you have to disregard a temporary improvement and the becoming is not in effect until the end of the interval since it's an improvent too. After 7 you don't need a take-off alternate since you have 600 meters vis = more than 550 meter required for CAT1.
In any case the weather is bad enough untill 7 o'clock that it doesn't matter if CAT3A is unserviceable or not since you will be below minima anyway. To cut it short there is no way around a take-off alternate if your departure is before 7 o'clock.
All of course assuming you have the best minima (no high mountains with consequent higher DA and hence vis requirements) and facilities.