After getting stuck in the A3 the other year I started to carry a pair of snow socks. If I do keep it another winter i am considering getting some winter tyres.
To save a bit of cash I was thinking i would only get 2 tyres for the front and put them on some 2nd hand wheels off ebay, really I am after some opinion if that is a sensible thing? Obviously I am not going to be driving it like i stole it, this is purely to help out a bit when snowy, I really see little benefit in getting the rears as well.
Another thing, I thought it might be sensible to go for 17" wheels and tyres rather than the standard 18"?
Bought some winter tyres a couple of years ago, when I moved to the grim north. Of Birmingham. Had snow each year since and it always seemed to hang around for ages, with a nice underlayer of ice. Fortunately had a spare set of wheels for the Leon already, so just bought the tyres. Got all 4, as I’m no driving god, so sliding sideways around corners at speed isn’t my idea of fun.
Have to say they are pretty spectacular, like driving in the rain, rather than snow, and much better when its just cold, and no snow. I had a picture on my old phone of my car with snow half way up the wheels outside work from earlier this year, and no problems. Be prepared to fall over when you get out though, as you forget that its as slippery as it is!!
If you are going to buy some smaller wheels, make sure they fit, and especially over your brakes first - there are no other wheels in VAG range, that fit over the brakes on my car, so not a lot of choice.
Just to echo JDS’s comments, really. I bought a set of German-made remould snow tyres three winters ago. Prices for known brands were, then, ridiculous for my Clio which is why I tried the remoulds…
They are TUV-approved and correctly marked, so are legal in any country where snow tyres are obligatory in the winter; the tread is based on Continental patterns and at around £40 a corner fitted and balanced, they didn’t break the bank!
Even compared to ‘all year’ tyres that I have on normally, the snow tyres were a big advance so I’d definitely recommend buying a set 
So other than possibly going sideways there is probably little to worry about, as for the brakes my A3 has small brakes so I could go 17 if it makes the tyres cheap enough to buy all 4.
Going smaller is ok, at the end of the day, the smaller the better. But I bet there’s a recommended size in your owners manual. At least then, your insurance company can’t have a wobble should the worst happen.
So, you turn in with your grippy winter front tyres and the rears go whooooaaaaa?? Seriously, if you’re going to go the winter tyre route, fit four not two.
This. You will be amazed at how much grip the winter tyres will give. And how little the normal ones won’t. If you remember that huge ramp going into JSR, summer tyres couldn’t even get up that in the snow. It must have been all of 20mm!!! It turned my car from something that was impossible to use, to not even thinking about it. And your Audi will be the same with lots of torque, fat tyres, and too little suspension movement at low speeds.
Been using winter tyres for 10+ years. There is no way I will ever drive November to mid April on anything else. Firstly as its the law and insurance is void otherwise, and secondly they are fit for purpose. Summer tires are NOT fit for purpose past 7 degrees. Its crazy that its not the norm in the UK.
You cannot fit only 2. The car will be more than entertaining to drive - it will be a guaranteed accident to happen. Buy them now, and they will be cheap. I just bought a full set and it was 60% of the normal price. The thinner the tire the better but if you get the same width its fine. The more weight in the car the better, so just keep the car full of crap during winter 
I echo all of this. They are great. Fit 4 though, what happens if you have to slam the brakes on at 80mph on the motorway! The last thing you need is the back end coming round.
Now going to look for a 2nd hand set of 17" wheels and then buy 4 tyres, it looks like the best option.
I fitted two on the front of the van and they worked great, I did not have any moments with the rear over taking the front even when I was dicking about trying too…
I see it like this, if your going to try and drive like you are in normal conditions in the snow just because you have snow tires on then you probably deserve to crash. If you treat the conditions with the respect they deserve then the snow tyres are a good aid to keep you moving when everyone else is getting stuck.
And just to really fuel the fire I only put snow tyres on the rear of the wifes bmw too as the only problem that had was traction off the line, I am not saying it would not of been better with them on the front but the idea was to get it home if we got caught out and they did just that when the year before it did not make it.
BMW’s should be left at home when it snows, snow tires or not. I just made sure ours was not curb side with the rest of them.
Boothy
I’d say if you are putting snow tyres on the rear then they must also go on the front for braking and steering.