Can someone educate me about violin and viola strings?

[MsFogi]

So I have been ignoring the requirement for dd to have a spare set of strings for various orchestras and she ended up having to beg an E string off a fellow player this weekend (clearly I will replace this). So...is there a particular make of stings I need to go for (or anything I need to be aware of when getting strings). I am going to need to buy a full set for both the violin and the viola so I hoping strings don't cost an absolute fortune (there seems to be quite a range on the web) - dd is grade 7 level (and clearly I have been lucky so far with strings holding up!!). I know absolutely nothing so am very grateful for any MN wisdom!

It depends on the violin, and on the kind of sound she wants to produce. At Grade 7 she probably needs decent quality strings (sorry! just be glad she doesn't play the cello or the harp).
Pirastro Eudoxa are a lovely mellow string, but quieter. They have a traditional gut core which gives them the softer sound.
Thomastik Dominant are synthetic, brighter and louder.
I've never used Corelli strings, but I think they're considered to be the better of the cheaper strings.

Before you buy, check whether she has adjusters on the tailpiece (the little screwy knobs). They can be taken off, but they do make tuning a bit easier. Eudoxa only need an adjuster on the E string - the others should be tuned with the peg. Dominants will be fine on all adjusters.

PS Her teacher should be able to advise which strings would work best on her fiddle - it's not good to mix brands, so you might have to fork out for a whole set to start with if she wants to change...

Thanks @Bubbleswithsqueak I'll ask her teacher - I foolishly assumed I could just order something on Amazon!!

You probably can order on Amazon once teacher has told you what strings they recommend. Top tip - order a set plus at least one extra E string as these go most frequently.

Also another tip, in case teacher has not told you. At least every 3 months, and preferably every month, do the following.

Put a plastic bag under the strings, around the bridge, being careful not to nudge the bridge. With a soft cloth and rubbing alcohol, wipe the strings to remove all the accumulated grot/resin. The plastic bag is to make sure no alcohol gets on to the varnish of the instrument. Cleaning does wonders for the sound!