It would be hard to recommend a bag because a lot of it really is personal preference. I think any high quality sleeping bag will probably work for your application.

1) Down vs Synthetic - You probably know the pros and cons of each in terms of weight, size, and performance. I personally like down, it's still the best insulator out there, and I'm willing to sacrifice the wet weather performance for lighter weight and smaller size. I'm still using the same down bag I bought 14 years ago, and it's still as good as any of the new sleeping bags on the market. A similar 14 year old synthetic bag would be considered be an old blanket by now. Synthetics are getting closer, and will continue to improve every year.

How much wet weather do you deal with? An ocasional rain shower in the mountains or are you out on in ocean storms for extended periods? Down is useless when wet, but I've had to sleep in a wet synthetic bag and it's not much better. If you think there's a high likelyhood of it getting wet, then synthetics are the logical choice, because it at least has some chance of drying in your lifetime. A down bag will perform well in damp conditions as long as you take reasonable care. The microfiber shells on the bag do a pretty good job of repelling accidental spills and or dunks. I found out while trying to wash my bag, I had a really hard time getting the insulation wet because the bag just wanted to float on top of the water. You really have to try hard to get it completely soaked though. Using the bag in humid conditions for long term is a differnet story.

Feathered Friends still make some of the best quality down bags. Stephensons Warmlite used to make some good ones, I don't know if they're still in business.

2) Temperature rating - You could buy a subzero bag for the coldest temperature you think you would possibly encounter, but if you want to use it year round you'll be too hot for 3 out of 4 seasons. I don't know what the average weather is like over there, I'd aim for the average winter conditions, and add some accessories for the extreme weather. A good insulating pad will help a lot, and if you're using a hammock, get two. I have a silk liner to extend the range of my bag, it works but I absolutely hate using it. I'm fine in a mummy bag, but the liner makes it feel really claustrophobic. If you do get one, find one with at least a half zip, or you'll have to wiggle yourself in and out of the bag through the top. A bivy sack will also lower the temperature range of your bag. For colder weather, a draft collar is really nice to have, but some people don't like the feeling of them. Also, the ratings for most manufacturers are just a guide, I wouldn't rely on them too much.

3) Cost is inversely proportional to size and weight. A decent down bag will set you back about $200-$300 (slightly less for synthetic), a really good bag could cost $400, and premium bags go even higher. At that point you're really paying more to save maybe an ounce or two. If you think you'll get into a situation where your life is going to depend on your sleeping bag, spend as much as you need. If a bad bag will only cause minor inconvience, I'd save some money and just get a decent one.

Also, a sleeping bag is only one part of the equation. Even the best bags in the world aren't gonna do much good sleeping outside directly in the snow. The mat and shelter is just as important.