I can't help you with questions #2 or 3 (though my friends with two have all said it is a bit harder, but definitely NOT twice as hard as having one). But as for your ? #1, I know a couple that went the adoption route in their mid/late 40's, and I recall them saying that they were not eligible to adopt in a lot of countries because of the age difference. They ended up adopting from Guatemala, and luckily found a child just before her husband turned 50, which apparently is the cutoff for that country. So, if you are considering it, I'd recommend talking to a professional soon and finding out the options.
Oh - and know also that the child you adopt, especially if you adopt a 3-5 year old, may likely have some developmental problems that will require extra attention from you, at least in the beginning. I know a few people who have adopted infants (9 - 15 months old), and the children were a little behind in motor skills and/or language skills, but caught up quickly once they received the love and attention they needed from a family. Of course, biological babies aren't guaranteed perfect either, especially at your age (sorry!!)