Even if you don't buy any of the specific boxes listed here, the criteria I discussed can help you make an informed decision
Karlach can also be paired with a background, which means you can go Boros or Gruul for better synergies in the beatdown department

Paradox Power Commander (2) The Thirteenth Doctor Yasmin Khan Creature (24) Dan Lewis Graham O'Brien Memory Worm Nardole, Resourceful Cyborg Sisterhood of Karn Thijarian Witness Jenny Flint River Song Ryan Sinclair Bill Potts Danny Pink Iraxxa, Empress of Mars Madame Vastra Osgood, Operation Double The Foretold Soldier Karvanista, Loyal Lupari Me, the Immortal Strax, Sontaran Nurse The Fugitive Doctor The Twelfth Doctor Clara Oswald Lunar Hatchling Flaming Tyrannosaurus Frost Fair Lure Fish Instant (7) Growth Spiral Surge of Brilliance Think Twice Beast Within Chaos Warp Flatline Decaying Time Loop Sorcery (14) Ponder Preordain Farseek Start the TARDIS Cultivate Impending Flux Search for Tomorrow Wreck and Rebuild Throes of Chaos Truth or Consequences Into the Time Vortex Last Night Together Quantum Misalignment Twice Upon a Time Enchantment (7) Carpet of Flowers Heaven Sent The Flux Become the Pilot Bigger on the Inside Fugitive of the Judoon Return the Past Artifact (10) Sol Ring Arcane Signet Psychic Paper TARDIS Talisman of Curiosity Talisman of Impulse Confession Dial Cursed Mirror River Song's Diary Sonic Screwdriver Land (36) Cinder Glade Command Tower Desolate Lighthouse Dreamroot Cascade Exotic Orchard Fiery Islet Forest x4 Frontier Bivouac Frostboil Snarl Gallifrey Council Chamber Game Trail Island x4 Mountain x3 Myriad Landscape Ominous Cemetery Path of Ancestry Reliquary Tower Rockfall Vale Rogue's Passage Rootbound Crag Sheltered Thicket Stormcarved Coast Temple of Abandon Temple of Epiphany Temple of Mystery Temple of the False God Vineglimmer Snarl Waterlogged Grove Plane (10) Besieged Viking Village Fixed Point in Time Kerblam

They can do a variety of different things, some can tap for two different types of mana while some have others can search for another land and others have special abilities
One of the most monumental sets to come out of Magic: The Gathering's recent history was the War of the Spark
Ignis Scientia Ignis is an interesting take on the Farhaven Elf type formula, trading a whole search for the ability to grab any land, along with the ability to make food tokens from whatever has been slain on the board