This is my first campaign so I am still learning the ins and outs of DMI just write the address, stamp, and not seal the envelope which might make it easier to open 

I am slowing learning all I can about Real Estate and have heard of 'yellow letter' campaigns.

I have began incorporating different marketing campaigns such as mailing yellow letters, building websites, squeeze pages, good adwords, etc... and have began to get those calls.

I'm still fairly new to investing as well... just got my RE license, preparing my first DM campaign, and a few other things to set everything up.

Work on a direct marketing campaign and target the area you want to have or move properties at, look for a mentor in the area as well, you will save thousands of $$$ in the future. 

But the leads come from the same marketing campaign...

I recently started a direct mail campaign in Washington DC, and I'll be visiting my very first lead's home next week to make an offer.

I had thoughts that "legitimizing" my business would be a smart first step.a.k.a. making a website, having business cards, getting a separate "business" phone/voicemail to answer prospective leads inquiring about properties or offering them.Would anyone suggest that this is the right direction to go before spending money on direct mailing campaigns/driving for dollars/running advertisements via craigslist or any other source?

The only thing I would add to it is that when you do go to your local REIA meeting, network to find a fellow investor doing the kind of investing your want to focus on.Offer to assist them with anything they may need (finding deals, painting their flips, organization, letter campaign's, etc) in exchange for their mentorship.

You can drive for dollars or start your own direct mail campaign.