Jeremy, welcome to Horizons Unlimited! We'll try to help as best we can.
First, what are your plans - are you at the start of a long trip, or?
Ideally, you'd get a bike licence and some rider training on a more suitable bike at home, plus get some good riding gear.
The biggest problem/question with the 900 you've been offered - where's it registered? If Nicaragua, taking it home to the US could be extremely difficult - registration difficulties as it's not a US import. If USA reg, then you need to be very sure it's legit, and you can actually transfer it correctly.
Of course another problem is it's not an ideal first bike! If it's a cruiser style it's a little better than a standard or sports style for a beginner as the seat will be lower, making balancing easier.
Getting a motorcycle licence in Nicaragua is unlikely, though you can certainly try! An adventure in itself I'm sure.
Insurance is probably not needed at all. US insurance is not valid in Central America. Usually when you enter a country, if they require insurance they will sell it to you at the border. I don't remember off-hand if Nicaragua does or not. Check the Border Crossings section under trip plan for information.
Travel by bike is the very best way to go in our totally unbiased
opinion, so we hope you do try it - BUT we want you to do it safely - Rider Training, an appropriate bike, and good quality riding clothing and helmets are HIGHLY recommended.
Read some of the stories in the travellers stories section - you'll find people who just got their licence and headed off around the world, and had a great time.
good luck, and keep us posted on what you do!
Seek, and ye shall find.
One world, Two wheels.