Go for Blaziken.
If you are going thru the trouble of getting a Speed Boost one, might as well use that one.
Also Blaziken should always have an Adamant nature if you plan to Mega Evolve it, to make better use of the huge Atk stat increase it receives.
The moveset remains pretty much the same tho.
Blaziken @ Blazikenite
Ability: Speed Boost
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Protect
- Flare Blitz
- Low Kick
- Stone Edge
Or you could use this one I found on its Serebi (I am the hope of the universe!)
Blaziken @ Blazikenite
Ability: Speed Boost
EVs: 96 Spd / 252 Atk / 160 HP
Adamant Nature
- Swords Dance
- High Jump Kick
- Flare Blitz
- Protect / Knock Off / Baton Pass / Thunder Punch
As for the double battles, it's a tough call.
A pokémon that could cover most of Blaziken's weaknesses while at the same time not being unusable without him.
This page may help you.
Hope this helped, and Welcome!