Custom Grey Knights?

[Waaaghpower]

So, I'm trying to go about creating a custom army of... something. I'm think Tau or Grey Knights, at the moment.
By 'Custom' I don't mean complete scratch builds, but rather basing off of the original models I would alter them to add a cool look.
So my question is... Well, what would you recommend? I'm thinking about heading back, way back to the roots of literal templar and crusades, using lots of fantasy parts and bits to give them a midevil look, but do you have any advice for how I could do that specifically? Places to find bits, specific models that could have specific conversions, (For example, I'd give a Vindicare assassin a bow and arrow with hunters garb,) tips on modeling, etc?

[Chocomel]

I use some Bretonian knight heads in my space wolf army to give them a HH era look.

[Chicop76]

I used a lot of dark angel bits for my Grey Knights. I also used a few fantasy models for priest. I picked up lok from forge world.

I think the sisters from necromunda would be cool.

For Tau I think eldar and necron bits may be cool. I did converted some guardsman into tau Aux before.

[SILK]

If you're looking for a medieval themed image to your grey knights, have a look at how the bretonians pull it off. It's not a case of reducing the technology present in your army (like the vindicare) but imprinting the imagery onto the models that already exist.

The main thing that comes from the knights of bretonia is that their armour is covered in a padded jerkin type thing. Copy that to your grey knights via the dark angels veteran kits ropes, and paint them in very individualist, very bright colours. Add heraldry to the shoulder pads, some Bret knights helms, and you're golden.

Just expand from there. Take alot of henchmen (consider taking Coteaz) to represent serfs and peasants. This will then allow you to include all types of mad things from arthurian legend, witches, lepers and the like. Your land raider isn't a thing of war! It's a mobile chapel for the emperor with some serious fire power!

[Chicop76]

I'm sorry. I think the sister models are from Mordihiem. I get Mordihiem and Necromunda confused. Mordihiem have some looks that would fit for Grey Knights.
http://www.games-workshop.com/gws/catalog/productDetail.jsp?prodId=prod1120119
http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat490067a&prodId=prod1120101


I think this squad would be great for female crusaders w/ thunder hammers. You will have to figure out what you want to do with storm shields, but I think they would make great crusaders.
http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat490057a&prodId=prod1120017

[Waaaghpower]

Actually, I'm considering flying to the opposite end of the deadlines spectrum and doing corrupted Sisters instead, so that might work well.

[SILK]

Traitor sisters open up a whole new can of worms. Be careful.