Only Noise Marine squads of 10 or more models can buy a Blastmaster, unfortunately.
I would advise getting some Bikers with the Mark of Slaanesh and Icon of Excess, accompanied by a Lord with a bike, the Mark of Slaanesh, a Sigil of Corruption, a lightning claw and the Burning Brand of Slaanesh
Also consider getting a Mastery Level 3 Slaaneshi Sorcerer: bumping up the strength of your sonic weapons is always fun.
I'd also make some Noise Marines with CCWs and BPs, to join a champion with a Doom Siren and Lucius as a designated melee squad (in a rhino with a dirge caster). Two Doom Sirens in one squad is a nasty combo, and if you want to charge with a unit with sonic blasters, they can't shoot. Better to let them shoot with bolt pistols and then get a bonus attack in combat as well for having 2 CCWs.
So you want two types of Noise Marine squads, basically:
- Melee Noise Marines with CCW+BP and a champ with Doom Siren+Power Weapon (power sword or lightning claw, I'd say). This gives you a unit who will usually strike faster than Marines, at I5, and who has an AP3, S5 flamer--again, great for killing marines. It's important not to put a power axe or power fist on the champion, since you don't want to be striking last when the whole point of Mark of Slaanesh is to strike first. If you want some anti-tank, give him meltabombs.
- Ranged Noise Marines in squads of 10 models with a Blastmaster and Sonic Blasters. Leave the champ naked: you don't want the unit to be in combat, so points spent for him to be killy are pretty much wasted. This unit, when standing still, can pour out absolutely massive torrents of fire: 30 bolter shots and an AP3 small blast template that all ignore cover. Nothing to sniff at, even if they only have medium range.
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!