you will need vanilla wow up 2 WOTLK b4 able to play catalysm

it is not really that expensive if you get the expansion gradually but yes a large sum if you get them 1 go

vanilla - lvls 1-60
bc - unlocks 60-70 + outland + draenei/blood elf
wotlk - unlocks 70-80 + nothrend + death knight
cata - unlocks 80-85 + worgen/goblin

so you can start with vanilla and make get your remaining expansions gradually, although you dont get to enjoy the additional race from the expansions tats all

the death knight class requires a lvl 60 character to unlocks so not much impact anyway