Hi,
if you do not have access to some indoor space. you'll need some small pens with a roof, (some of ours are just pallets, with tarpaulin tied on them) to put any weak lambs or twins that have not yet bonded properley. Depends on your sheep, but a good dog is always useful, especially if it will catch a sheep for you.
Lambing equipment wise, the same as for indoor lambing, but I always carry a dog collar and lead, incase you need to restrain a sheep, you can tie it to the fence, or hay rack, for example, if you need to get a lamb a suckle, it's just so much easier, than trying to hold a sheep and get the lamb to drink.
And finally at least two, preferably three sets of good waterproofs.
Regards
Sue