I'd try just doing a starter kit first.... say 1-200. My question would be how to divide up the payments so that neither you nor the buyer felt yourself to be at risk. I wouldn't do "special" patterns.... just the basic power levelling package.
As for best profession to start with... I have an instinct that blacksmithing is the most intimidating to me so I'd say that might be a good place to start. Especially if you can do it for a nice round number of gold that doesn't sound intimidating.
Scantron: Interesting idea. I might "stage" the available kit, for example, from 1-200 charge "x" gold, then from 201 to 350, "x", etc.
AlexS: HEY - that looks like MY guild bank tab!
So you guys would NOT provide the vials? And what about COST - what could something like this go for? I'm thinking an "x"% above the value of the herbs, then a fee for the farming time. I LOVE the idea of the deposit - then the guild rank, great idea.\
Using the Undermine Journal (quite possibly one of the best sites for AH data EVER), I calculated the AVERAGE prices for every Alchemy Mat over the past 2-3 weeks on my server. It's $4k and change. So would charging 6k be a little too steep? Some of those Herbs are NOT easy to farm!