If I buy the standard version of a DVD collection, would I be able to buy the extended version of individual instruments via download? 

no, you have to choose between dvd and download instruments and stick with it.

It has to do with licences. Dvd products are cheaper because it costs vsl only one licence,

download instruments are more expensive but you get a seperate license for each instrument,

this way you can buy extended libraries for only single instruments. hope this helps

Well, they're also cheaper because you're buying many at once.  It would be nice to just forgo any extra licence to only get extended for one or two instruments...