Big Island is about the only affordable one for your budget. However, not all land has soil on it, especially over in the Puna/ Pahoa district, so don't buy land sight unseen.
You're much more likely to be able to buy land in a rainforest than somewhat close to a beach. The side of the island with the soil doesn't have the big white sandy beaches, more of a rocky beach. The big white sandy beaches are in the resort area, don't have decent soil to grow anything and are priced way out of your budget range anyway.
North of Hilo if you can manage it has a lot less crime (mostly "just" theft, but a lot of theft in Puna/Pahoa) and there is deeper soil on the Hamakua coast.
Hawaii Island has loads of different micro-climates, depending on what crops you wanted to grow, you may prefer one elevation over another.
The University of Hawaii College of Tropical Agriculture and Human Resources (called CTAHR for short) has more information about growing things in Hawaii than you'll be able to dig through:
http://www.ctahr.hawaii.edu/site/ Sending "exotic" stuff to the mainland or elsewhere involves red tape and shipping, there's farmer's markets here which would be an easier entry into sales than boxing and shipping.
Up around Mountainview to Volcano area might work if the Hamakua coast doesn't suit. But before buying land, have you visited the islands before?
