Spoilers for the The Magician’s Apprentice S09E01 (story 251) below.
Out of all the possible gambits, why did Missy think that this deal would be attractive to the
This is the plea she makes:
Missy: And you know all about those, don't you? This is a TARDIS. With this, you can go anywhere, do anything, kill anyone. With this the Daleks can be more powerful than ever before. You just need one thing...
Clara: No. Missy, no!
Missy: Me! You need me. A Time Lady, to show you how it works. With this and with me, everything can be yours. And you can burn it all, for ever and ever and ever. Or would you rather just kill me?
From Utopia S03E11, and later in story 187, we know that Missy can use the TARDIS, so her ability to fly it isn't in doubt. But the benefits a tame Time Lady flying a TARDIS offers a Dalek, they already have:
• The Daleks can Time Travel
The concept of the Time War was that the Daleks could time travel and have since done so numerous times in modern Doctor Who in episodes such as Victory of the Daleks S05E03 (story 205) and Daleks in Manhattan S03E04 (story 182).
• The Daleks can transport troops through Time
Since Resurrection of the Daleks (story 133) have had Time Corridor technology which is shown to be adapt at troop transport across time, and seen as recently as an unfollowable escape route in Victory of the Daleks S05E03 (story 205).
• Dalek Time Travel is at a par with Time Lords'
Previously Dalek time travel was shown to be not as precise as the Time Lords', but Dalek Sec was able to navigate into the Time War as revealed in The Stolen Earth / Journey's End S04E12 / S04E13 (story 198), so around the time of the Time War and after they're technology was comparable to the Time Lords.
• Dalek spaceship design is advanced
As seen in Army of Ghosts / Doomsday S02E12 / S02E13 (story 177), during the Time War the Cult of Skaro was able to create a Void Ship, something the Tenth Doctor declared as impossible:
Doctor: Well, it's impossible for starters. I always thought it was just a theory, but... it's a vessel designed to exist outside time and space. Travelling through the Void.