Hi Nats.
December can get cold up there! But there's plenty of fake North Face Stores to buy extras 
7-10 Trek? Hmmm. If you took a flight to Jomson you could trek up to Kagbeni and then back down to Gorepani/Tadopani. Or Simply trek up to Jomson and fly back down again.
Annapurna Base Camp 8-10 Days, great end view, sheer ice mountain. But just a valley on the way there.
First up with the trekking companies, Don't go to the ones in Kathmandu. Most of these treks take place from Pokhara, which is a 7 hour bus ride, or short plane ride from Kathmandu. It's a great place in itself too. It's a lot, lot cheaper to get a guide from there.
Having said that, there are guides and agencies all over the place. So there's not problem in booking stuff out until you get there.
There's a lot of this information on my site if you go to my profile you will find the link. Also, if you need the name/contacts of the guide I used just send me a message.