For me there is only one winter bike to buy and for a change its one of the cheapest!
The gearing that I ride on mine is 48×17 which in old money is 74 inch, now for me being a powerful cyclist who typically rides at a slower cadence then this is fine however for some this may be a little too high so I would recommend starting with a slightly easier gear ratio and maybe work up from there.
During your base mile rides on your single speed you get a variety of skills that are the cornerstone of every training plan and you will be including them during your sessions without actually trying. These include speed skill drills which will happen when you are pedalling down hill as your cadence will increase to over 100rpm as no option to change gear!
On the other end of the scale going up hill you will be using muscular force due to having to reduce your cadence to as low as 50rpm just to get up it especially if you are targeting a low heart rate.
Long rides will give you the endurance so that is the 3 corners of the triangle covered in one ride and with single speeds being cheaper to purchase why would you consider buying any other winter bike?
One note that I must say is that you will find they normally come with a flip flop hub so you can ride it fixed (no freewheel) or single speed where you can freewheel if you want and that is what I would recommend unless you are confident with that type of riding? Falling off will do nothing for your winter training plan!