I wouldn't make the upgrade cost that much if at all. These are the only walkers a new player will have access to feasibly. I would tune them to be more functional for a new card base. It was really frustrating when I chose to focus on Liliana early on (because she's my paper magic favorite), only to find that a deck based on discard won't work with her without specialized cards to back up the strategy.
I can't speak reliably toward a lot of the other walkers for a new player, but Gideon is who I turned to after Lili failed me. He worked actually quite well for the purpose, and he still seems relevant but not quite as powerful.
Lili could use a tweak to numbers and her first ability
First ability:
lvl 1: Discard your last and their first
lvl 2: Discard your last 2 and their first and last
lvl 3: Discard your last and their first and last
lvl 4: Ability costs 6
Original ability doesn't really harm the opponent that much early on because they don't pump out their cards fast enough anyway. This makes it more relevant. Not sure what I'd do with her other abilities. Maybe just decrease the cost on the ultimate so that you can more feasibly use her first then her last if you have enough.
My 2c