The 400g merino is too expensive and heavy for it's warmth. Merino shines in the baselayer weight, because of it's comfort (subjective) and anti-odor qualities.
Some people do fine with synthetic baselayers. They are cheaper, lighter and dry faster. I just can't stand having it against my skin for more than a day, or when I sweat in the shirt and it dries. Unwearable after that for me. You may be different. Your blend t-shirt may work great for you and be best of both worlds. There have been claims of people wearing the same merino shirt for 3 weeks and being very comfortable. From personal experience (though not 3 weeks), I can believe it's true.
Everyone has different opinions, but if you read enough of these threads a common theme emerges: the lightest, warmest, most compressible piece of clothing is a down sweater/jacket.
The only problem with down is if moisture accumulates (multi-days in a humid environment) and loft is lost. It will lose it's warmth. It's never happened to me, but I haven't been out in a weeks rain etc, your call. For every down insulated sweater/jacket, there is a comparable insulated synthetic equivalent (it's just a little less warm and bit bulkier).
The list that "S. Long" gave earlier in the thread sounds very good.
If you want to play it safe, get grid fleece (Patagonia R1 or MEC T3) and a Montbell Ex-light or Montbell UL Down Inner, one hooded one sweater and think layers as suggested. Remember, you'll have WPB rain gear too.
Check out the Montbell line (various warmths) for insulated gear, whether down or synthetic. Pretty popular. They fit a bit short, so keep that in mind when ordering.
or the Eddie Bauer First Ascent line. Usually fits long and bigger. Sometimes they have crazy sales. Unparalleled return policy.
Good luck, hope this helps.