All right, I think I've made a decision. I've got some Rowan Plaid yarn in the stash. It is very soft (42% merino, 30% acrylic, 28% alpaca) and I have just noticed it needs huge 8mm needles! 3 stitches per inch! That settles it. Yarn that fat promises quick results.
I've also just measured my ratty old black sweater and discovered that it is a massive 44" chest circumference! No wonder it is cosy. It would never have occurred to me to plan a sweater with that much ease, so I'm glad I measured! I think I will make a swatch which hopefully will also be the first sleeve. Unless I decide to knit in the round from the bottom up.