Hi Lisa,
Congratulations on the new caravan, hope all goes well at the handover.
The question of what type and what size of gas cylinder is one that is raised quite often. Different people have different view points.
Some only carry one cylinder, to cut down on weight, and change as necessary. Fine as long as your supply doesn't run out at an inconvenient time ( sod's law states it will). I always carry two, a lot of others do as well.
Regarding what type, you are quite correct in that red (propane) can be used at much lower temperatures than blue ( butane). For years I only used butane, only ever had a problem once but then I tend not to caravan from late November to early March.
Size of cylinder depends on personal choice and space in gas locker box. If using butane most folk go for either the smallest size ( approx 4.5kg ) that weight is the weight of the gas not the combined weight of gas and cylinder. If memory serves me right that combined weight is 10.4kg or thereabouts per cylinder. There is a larger size, think it is 7kg of gas, more economical to purchase but more weight to carry around. I used to use two of the small 4.5 kg cylinders.
Several years ago I switched to calorlite cylinders. These are propane, so suitable for year round use, and in light weight cylinders. A 6kg calor light weighs in at the same total weight as a 4.5 kg butane. That would be my recommendation.
I know some use makes other than calor. I have no experience of them but I would look at how readily available replacement cylinders are. Most if not all caravan sites will stock calor refills but I do not think that is the same for other makes.
Some info here:
http://www.calor.co.uk/outdoor-living/caravanning/