Skaven, Araby, Lizardmen, Vampire Counts and Daemons in particular don't come up much at all except the odd blister so I'd just bite the bullet and buy what you can afford from GW before the ranges all vanish one day.
High Elves, Chaos, Empire, Orcs and Tomb Kings in particular you should be able to get deals off e bay if you are willing to be patient.
The other armies are 'in between' and you might doa bit of both?
I'd just go for whatever army floats your boat, figure and background fluff wise.
Welcome to the best game GW has ever done!
Dave