It is hard to tell you which weight to use. I would say the 2.5 ounce if walking but maybe you are chilly and would want more.
The M90 fabric is good. It is much easier to work with than the M50. The M50 is thinner and you will save weight, but I decided the weight saved for a vest or something of a similar size was not worth the cussing. There are some threads with tips about sewing really thin material like M50, if you decide to use it.
A vest might be a thing to think about. A vest under a shell is warm. If it is chilly enough to want a vest, I would want something on my arms under that shell and would wear some kind of long sleeve shirt rather than the t-shirt.
I don't hike/work in my vest, but I do wear it under a wind jacket while walking around and I like it. The wind jacket has a hood and the head has a wool cap. Fine for me strolling in the low 30's. Maybe fine hiking in colder temps.
A vest is easier to make than a jacket. If you have sewn much before, that won't matter. If this is a fairly new hobby, vests are a good starting point.
There are threads with ideas about accounting for the extra length when quilting and etc.
PS - If I remember correctly, the M90 is more breathable than the M50... I don't have either with me now, but I think I could draw more air through the M90. This was using the sea lamprey test. Someone will correct me if I am wrong.