If you can find a dictionary designed for youngsters, the definitions will be simpler and less kanjified. I don't know of any electronic ones, so you'd probably have to fork out for a paper one and deal with manual lookups and limited entries.
If you're using a real dictionary, then you need to be able to read kanji and have a fairly decent vocabulary. If you've done something like the core 6000 from smart.fm or Kanji Odyssey, you'll probably be fine. Try easing yourself into it if it's too difficult. Only look up words with fairly simple English translations, or ones that don't involve difficult concepts. Reading definitions is a form of exposure in itself, and you'll find that it will help you learn new words.
I have no idea how many words I know, but it's probably between 6-9k and I've being adding
goo definitions to my flashcards for a good few months. I still include the English to get an immediate sense of the word, and then read the J-J for a more accurate explanation. You start to see how inadequate J-E dictionaries are when you can compare them like this. I include a couple of example sentences for a bit more context as well. There have been many occasions when a word in my deck has appeared in the definition of another word, which really helps to solidify it, as well as making the definition easier to understand. I often add words from the definitions too, so it's also a good source for new vocabulary.
Like I say, even if you don't fully understand the definitions at first, they'll become clear after you learn more words, so it's definitely worth adding them.