Hey lordcardigan, l found your post
If you're into Alpha Legion fluff then yeah groups of cultists are mandatory and a sorcerer can give nice buffs to your army on the cheap (if he's taken with minimal equipment).
To start helping with your painting skills l'd recommend looking at the "Warhammer TV" YouTube channel.
They've got great 1 to 2 minute long videos on painting things from armour chipping to muzzle burns while being easy and clear to follow along.
Lucky for you they got a painting tutorial video dedicated to the Alpha Legion:
A great tip for painting is "wetting your paints," you'll hear that a lot in many painting tutorials and you can't go wrong with using a wet palette, whether bought or homemade.
Great to hear from you!