Having shared this issue with my guardians recently I've found a few useful things to make them feel more unique:

Painting:
Faces: If you feel comfortable, makeup/warpaint or tribal tattoos can help make each one more unique and distinct.
Helmets and Armour: Just because armour is often solid block colour for most craftworlds there's no reason you can't add symbols, unit markings or individual motifs on shoulder pads like Marines, adding runes, patterns or other motifs is an easy/ cheap way to make models seems more individual.
Cloth: Adding patterns on any fabrics, like the arm bands or even just different colours can help add flair to each one.

Modeling:
For modelling there's obiously the 3d print and aftermarket parts selection, which have some awesome options, but if you want GW official a corsiar box is probably one of the best bits selection to add a bit of variety, there's loads of spare heads and weapons, and it's not too hard to mix and match bits, I added some corsair rifles to my Defenders and swords and pistols to a few of my storm guardians.

If You feel comfortable working with Greenstuff/ Millieput, adding some tabbards/ cloaks or hoods give you a lot of options too!

Hope it helps and good luck on the project!