Hi Susie We've been to Australia twice, as my son lives there now. We like Singapore Airlines, great service and you get well looked after compared to some. The plane had space at the ends to stand around and walk a bit, even stairs to the next level, so we could exercise our legs. It wasn't too cramped, I suppose it depends how tall you are. You can go on seat guru to find exact space on various planes.
Drink lots of water en route as flying dehydrates you. Moisturiser and lipsyl also handy.
We had time limitations and didn't want to stay several days somewhere en route, though that would be best if you have the time.
We had a stop over at Changi Airport in Singapore, it is a wonderful airport compared to many others. We only had about 12 hours, so we had a free bus ride round Singapore, and then booked into the Transit Hotel which is in the airport. It is basic but clean, and we were able to get about 6 hours proper sleep before continuing our journey, which really helped with the jet lag. Another time I wouldn't bother with the bus tour, we were all too exhausted to enjoy it. I'd just have a good sleep, shower, then wander round the airport - they have a wonderful 2 level butterfly farm among other things.
You could ring the airline and ask in view of your husbands condition, could they offer you a more spacious seat. Sometimes works. I'm sure you'll have a wonderful adventure. Have fun.