choosing the suitable focal length really boils down to personal preferences.
what works for me, may do the otherwise for you. i dislike walkabout with 50mm on crop, so there, i say it sucks.
what lens do you have now? maybe you can set the lens(assuming a zoom one) at those focal length which you mentioned and try shooting photos to see if it's workable for you. else you can try renting those lenses from shops.
for the 70-200 f4L, my experience tells me that you will need good light in order for the lens to perform well. go for the IS version if possible. other than that, IQ is stellar, nothing much to complain about.