5 litre demijohn - none, but add it below the shoulder, you need to leave a decent amount of headroom for it to froth up. cider/wine are less vigourous than beer so you only get a little action, but get the juice to this level here for fermentation;
after that, if you want to maximize it, you can top it up to below the neck with apple juice and it will ferment again, but a lot less vigourously. you will realistically fit 4.5L in a 5L demi john.
temperature, room temp is fine. 18c - 24c is best for most yeast.
basic apple juice is a-okay! it will indeed ferment to around 4.5% - 5.5%. Don't be excited by making it mega strong with extra sugar, it will take longer to ferment, condition and mature - the weaker it is, the quicker it will taste as good as it will get.
we have some good recipes for Turbo Cider, Vimto Turbo Cider, Wurzel's Orange Wine...they are forum favourites and we love making them because they taste great! I highly suggest using tannin, citric, malic and pectolase - they will all benefit your cider massively and most are available from easy sources.
If you want nutrient, add 1tsp tomato puree, that'll give the yeast something to feed on!
