I've been receiving post cards from Parrot Home Buyers.  

I'd love to hear your thoughts on what kind of mail typically works best (typed letter, handwritten yellow letter, postcard, etc.), how can I quantify the amount of repairs the house is going to require (I have an idea of the ARV by looking at comps in the area, but I wouldn't know what to offer because I haven't seen the inside of the house and don't know the repairs required), and any other advice that you may have.

I got it from sending post cards.

I would be surprised if it had the expiration date for someone that was not the listing broker (because pre-sending out expired postcards would be real smart).

And I'm not talking about "fake eviction letters" or "postcards" or things like that. 

Selling quickly at the first sign of dollar signs is something REI ignorant people do when they receive a "We Buy Houses" postcard in the mail. 

Craigslist ads (post about twice a day and reply to every for rent post in your target area), Direct mail (send letters/post cards to every foreclosure filing in your county and then follow up with them again about 3 weeks before the foreclosure sale is set to happen), Drive for Dollars (when you see a house that is in need of repair, knock on the door, ask about the owner's plans).

My plan would be to identify zip-codes with most cash transactions in markets with median home price at or below 320K, then mail at least 1K post cards every week for about 6-7 weeks.

They'll even help you with marketing materials like postcards or door hangers.

Anthony,I suggest you come up with a post card and letter rotation that will spread your budget out over time but keep you in front of them.