This is the sort of business I have considered going into - its ok, I am miles away from you, if I ever do it!!!
We are short of smallholder supplies places round here, and the agricultural merchants only stock basic things, and then not many of them. On my travels (I go all over the place showing horses) I have seen a lot of farm shops, selling fruit veg and the like, and incorporating a tea shop, cafe etc. and they all seem to be very busy at weekends. Farm gate selling seems very popular. On the other hand, I was talking to a man who has a garden nursery in a rural area, and he said he was struggling with the credit crunch, and it was only his livestock, sheep and cows, that helped him keep going.
Christmas trees - a lot jumped on this bandwagon thinking it was a good idea. My OH works as a farm manager, and last year he planted thousands in two hill fields. They had a flock of shropshire sheep which supposedly are the only ones not to eat these trees - to keep the grass round the trees down - although I was not convinced. Anyway, only two ewes left now, as they ate the trees, and most died of liver desease. Also some deer got in, and helped themselves too, but lived to tell the tale. Trees have recovered, but my OH is now lifting and potting them to be sold on.
Apparantly, whoever sold the trees in the first place say they will buy back when bigger, so maybe there is money to be made there, if you can keep deer etc away!!
I do think what you are wanting to do is a good idea, but maybe start off with a few things, and see how it goes. You can of course add to it, or get rid of anything not selling.
I wish you luck and hope your venture is successful.