if you want fast attack for deathguard then spawn are for you

. However, if you want fast attack you could move away from deathguard and do another scheme. I'm not sure about the purge and their tactics so you could paint up a nurgle biker list in their colours?
How about this list:
HQ
Chaos Lord-MoN,bike,daemon weapon=180
Troops
7xPlague Marines-2xPlasma gun, Champ w/power fist=231
7xPlague Marines-2xPlasma gun, champ w/power fist=231
7xPlague Marines-2xFlamer, champ w/power fist=211
Fast attack
7xChaos Bikers-MoN,2xmelta gun,champ w/power fist=341

6xChaos Bikers-MoN,2xmelta gun,champ w/power fist=308
Total=1502pts
The chaos lord and bikers flank around the back of the enemy lines while the plague marines seize objectives close to your deployment zone. The bikers then proceed to turn your enemies transports into sludge then the plague marines mow down the survivors. The lord and his bikers (the smaller unit) can also target MCs with their two strength 8 AP 1 shots the power fist and the nurgle daemon weapon (which wounds on a 4+). Towards the end of the game the bikers can then make a last turn dash to contend enemy objectives. This is a very tough list (the minimum toughness is 5!) and can do a lot of damage but will be out numbered. The bikers will get every gun the enemy owns pointing their way in order to down them so remember to always move the bikes at least 18" in order to claim their cover save so basically they're either moving to fast to hit or smoking a tank. TARGET THE BIGGEST GUNS FIRST! I cannot stress this enough. You need to make sure that the enemies battlecannons/basilisks/etc. are out of action before your bikes slow down because if they don't have their cover save these guns will obliterate your extremely expensive units in a single turn of fire. When faced with ork or nid armies switch the meltas for plasmas (drop the PF to a PW on the other squad to make up the points) and send the bikers MC/elite unit hunting.
Hope you like the list. It will take a long time to get used to but will work. Bear in mind that an intelegent enemy will try to drag you down with weight of shots and 20 bolter shots will drag down an average of one bike per turn.