yes you definitely have to reach the middle comfortably. in the gardening books it tells you to dig up the turf sods and turn them upside down then fill up with good soil. we have lined the bottom with cardboard, and weed fabric before to stop weeds coming up. wev also had solid wooden bases which is the best way but takes up more wood.
soil type depends on what you want to grow, sandy soil would be good for carrots, and they grow well in raised beds.
i have grown tatties, onions, salads etc in raised beds but its easiest to choose crops that are easy to harvest with hand tools. i prefer to dig up tatties with a big fork. also mixing in manure takes a bit more effort with hand spade.