I find oils to be MUCH easier to work with than acrylics and I was first taught to paint in acrylic. I don't think oils would necessarily be quicker though.
I wanted to comment on your marketing idea of creating a postcard of this image. I don't think it would be tacky at all and in fact I'd highly recommend it. Check out Modern Postcard (at
http://www.modernpostcard.com ) They're fast, cheap and do good quality work.
As for what to put on the postcard, I'd make the finished image almost fill one side of the card, with a small area that has your name and something about the fact that these are paintings. People assume that you're a portrait photographer unless they look at your postcards closely.
On the back put your contact info and a space for mailing info (Modern Postcard has sample layouts) and a bit of info about prices. Something like "Prices start at $....." Then clients can contact you for more information.
Use these postcards as your business card, give them to everyone you know and you're on your way!