7.3 is a great engine but don't expect power like the newer diesels. It will do fine if you stay within the ratings but don't expect single digit 0-60 times pulling 10,000lbs.
$20k for a 2000 is a bit steep. Not familiar with the Arizona market but I would expect half or less that price. Hopefully it's an desert truck so the body won't be all rusted out. For $20k, it better be perfect.
PS: I would want an explanation for why it only has 87k miles. It could be an old guy who pulled the RV to Arizona and back once a year or it could hint at something fishy.