In late July we'll be coming into King's Cross Station with a fair
amount of luggage. Ideally we'd like to take the tube from there to
Southfields. What we haven't been able to determine from the available
information is whether we'd have multiple stairs to descend and climb
to do so (I know we'll have them at Southfields). I'd welcome advice
on what we'll encounter and whether we should just gulp and take a
taxi.

It seems to me that there are two possible routes
1) King's Cross - High Street Kensington - Southfields
2) King's Cross - Victoria - Southfields

Is either of these more or less a problem - it looks online as though
Victoria has more escalators?

Also, my research seems to indicate that an "oyster card" is the smart
thing for us to get for tube and bus travel during our week in London.
Does that make sense? If so, can I buy one at King's Cross?

My thanks for whatever advice you can give.