Hmm... well, I've given myself a crash course in wheels and tires the last few weeks. I'll try to condense what I've learned...
<<Sorry it turned out long. This seemingly simple subject has lots of nuances.>>
Well... I think the first thing you're going to have to decide is: are you going to have 1 or 2 sets. If you're going with 2 sets you'll probably want to get another set of rims. (It would be possible to keep removing the winter tires and mounting the summer tires, but it's understandably not good for them.)
Personally, I decided to go with a single set. It sounds like you'd be better off with a dedicated winter set.
Next, decide what is safe. Although I do not have much personal experience with this, people agree that summer tires are not safe in snow/freezing temperatures. If you're going to do a lot of driving in snow you'll probably want to get winter tires.
You'll also want to consider tire size. This will give you a good idea of what sizes will fit:
http://www.tiresizecalculator.info/ (By the way: the stock tires are 215/45 18... in the U.S.A. at least.)
If you go to the tire-store websites you'll notice that tires for larger rims are more expensive. If you're buying new rims opting for smaller rims NOW will save you a decent amount of money now and in the future. The MS3 comes with 18" rims, it seems pretty much every 17" rim will work, and even
some 16" rims will work.
Keep in mind when mucking with the diameter of the tire you're going to change the speedometer and odometer accuracy. I imagine it'll change the performance of the car some too. Wider tires will give you a larger contact patch (the area of the tire that actually touches the road). 'They' say for winter tires: narrow is better.
For the specific brand/model of tire you're at the mercy of manufacturer specs and reputation, owner testimonials, and professional reviews. From what I've seen most tire websites have compiled owner reviews. TireRack.com also reviews and rates some of their tires. Although it sounds like it would be wrong for you, here's what I chose:
Pirelli P-Zero Nero M+S.
As for the rims, as I said: lighter is accepted to be better. ...So long as the wheels are strong enough lighter certainly wont
hurt performance. After the diameter you'll want to take the width into consideration. Tires have a recommended width, but can be mounted to wheels within a range of widths. In my slightly-informed opinion 7.5" is probably what you should shoot-for when looking at performance wheels. 6" or 6.5" might be good for winter wheels.
Once you've chosen your desired wheel diameter and width you'll want to make sure the
offset,
center bore, and
bolt pattern are compatible with your MS3.
Here's some numbers to use as a guide...
Offset: 45mm - 53mm (A smaller offset will cause the wheels to stick out more.)
Center Bore: 67.1mm (wheels with a larger center bore can be adapted (usually the adapters are free), wheels with smaller center bores can be "bored out" by a machine shop... I imagine they want a pretty penny to do this. Also it's possible boring out a wheel will weaken it, and if done wrong could cause the wheel to mount off-center which would be bad. You'd really have to be in love with a wheel to have it bored out IMO.)
Bolt Pattern: 5-114.3mm (also sometimes called 5-4.5in)