If you buy the first and second game, you'll get to play the following races in Mortal Empires:
- The Empire
- Dwarfs
- Greenskins
- Vampire Counts
- High Elves
- Dark Elves
- Lizardmen
- Skaven
The following races are all DLC:
- Chaos Warriors
- Wood Elves
- Beastmen
- Norsca
- Tomb Kings
- Vampire Coast
There's also a bunch of Lord packs, which unlock Legendary Lords and some extra units for the campaign. Some of them are free.
There's a lot of content without buying any DLC. And all of the DLC is in the game whether you buy it or not (buying the DLC just unlocks it for your use) so you don't really
need to buy any. But if you're spending £23.58 on the two base games, you may as well round that up to ~£30 and get the £5 off. If there's nothing else you want on Steam, nab The Prophet & The Warlock plus The King & The Warlord/The Queen & The Crone/Norsca, or either of Vampire Coast or Tomb Kings.