I have a rain jacket and a poncho, but feel that i should invest in a good quality Jacket?
my practice, developed over many years to to take four layers.
the outer layer should be rain and windproof and, most preferably, have a breathability factor. in my part of the world this is called a parka and weighs under 200 grams (? 7 or 8 ounces ?)
The other three layers are relatively lightweight merino tops - two short sleeve and one long sleeve - about 130 grams (? 5 ounces) for the shorts and about 180 grams (? 7 ounces) for the long.
For normal weather starts I wear a short sleeve top.
If a bit colder, I put on gloves, but otherwise bare arms.
Nine times out of 10, I have warmed up quite nicely within 20 to 30 minutes. That 10th time may take a little longer. And I get some experience for next time at that temperature.
If especially windy, the combination might be a short sleeve top with parka (with the sleeves pulled up to the elbows) with hood. This keeps the upper body warm and does not overheat the lower arms.
With this combination I would also leave the front of the parka undone, unless especially necessary.
Using various layers means there is a small air buffer between each layer that will help your body to warm up as required.
So, apart from the parka, no special one-off garments. Others above say much the same.
In normal conditions, with one short sleeve merino top worn, the weight carried is about 500 grams
For below the waist I have a rain kilt, with elasticated waist, that weighs about 100 grams.
While I have used the very popular knee length rain coat that also covers the pack, I had too many incidents not managing to put it on in windy weather that I have, reluctantly, put it in my bottom drawer.
Having said all that, I acknowledge we are all different in our needs and experiences.
For example. in normal day-to-day living we look out the window (or watch/listen/read weather reports) and put on what seem best from our wardrobe.
For long distance walking it is not practical to carry the range of stuff in our wardrobe.
I simply recommend you put aside "normal " and experiment with what will work for you to carry for a month of walking.
Kia kaha (take care, be strong)