I want to buy a new set of bib tights for cold weather biking, but there are many choices and variations and huge variations in price. What qualities should I look for - e.g. fabric, padded insert, straps, back material, water resistance.
Some have a section of material on the back but would these models only be for more extreme cold weather riding?
I consider cold to be 5 degrees celcius to -5. Extreme cold -5 and lower. Majority of my cold weather riding is not extreme.
This is a somewhat subjective topic as everyone has their own personal preferences.
But for me (a skinny 30 something riding in Scotland where winter is typically -5 to +5°C):
Fabric: Generally a thick 'roubaix' style brushed lycra for warmth. Windproof fabrics tend to be less flexible/comfortable and I would only use them in extreme cold weather.
Padded: No. You can wear your existing shorts under non padded tights. If they have their own pad, they need to be washed every use, without a pad the tights can be re-worn after some rides.
Straps: Personal preference. Some prefer the higher waistline and extra material for warmth. Personally I prefer easier natural breaks as riding in very cold conditions makes them a more regular occurrence.
Water resistance: I find DWR treatment an amazingly good feature on tights, with splashes from puddles just bouncing off, keeping you much drier and warmer. Softshell type materials (which are also usually coated) will be even more water resistant, but have their own drawbacks (covered above).
Additional points to complement Andy's answer.
Length - enough of it so your ankle cuffs are lower than the top of any shoe covers you wear. Otherwise that strip of exposed ankle skin is going to get very cold.
Fit. I have several pairs of tights/pants, and the looser they are the worse they are. They should be quite snug around your legs all the way down, but especially so at the upper thigh. You also want minimal friction between any skin in the crotch and genitals, and your legs, specifically the inner thigh.
Storage - very few bike pants have any sort of pockets. Personally I wear my normal pants over top of the tights, which gives me a belt and pockets for keys and wallet and cellphone.
My warmest tights are almost furred on the inside, and are significantly warm. But they were very cheap, have poor fit and zero water resistance, and minimal wind resistance.
I've also worn tights under waterproof overtrousers, and this can work well or can parboil you while riding.
YMMV, so try and have several options for different levels of weather.
Stirrups or non-stirrups.
Some tights have stirrups - strips that go from one side of your ankle, underneath your foot, and to the other side of your ankle. They prevent the tights from riding up your lower leg.
They also prevent you from putting the bottom of the tights over the top of any boots, shoes, or shoecovers you may be wearing. Putting the lower end of the tights over the top of whatever is on your feet is a useful means of delaying your footware from filling with water when it's wet outside (note I said "delaying" and not "preventing"...).
I'd like to suggest wool tights instead of Lycra. Wool is quite breathable and helps you stay warm when you inevitably get damp. Medium weight wool is good for me from 0-12°C. You'll want zippers down around the ankles to facilitate removal with your shoes on.
It's also useful to have a rack and bungee cords for extra layers -- either carrying a little extra in case the weather goes sour, or for the layers you take off as the weather improves.
