I am completely new to jetskis as well. Been boating for years, but never had skis of my own.
I just purchased two 3up sparks (no ibr, no convenience package) and a double loadrite trailer for $16,100, so I would say you are getting a decent deal. Since you are getting a 2up and not two 3ups, you can try to get them to drop it down a couple hundred more.
Dont forget, you will have to buy registration stickers ($20), PFDs ($35-$125), and fire extinguisher ($25). Possibly you can get them to throw that into the deal?
When you get your skis and trailer, I would recommend practicing driving. Go to a big open lot, practice turning, backing up, etc. You dont want to get to the boat launch on the first day and look like an idiot. You'll get the hang of it quickly with a little practice. Remember, small adjustments with the steering wheel. Always be mindful to take wider than normal turns.
Once you get the skis in the water, follow the break in procedure that seadoo recommends. 5 hours of 50%-75% throttle max during this period to break in the engine. Never go full throttle in the first few hours. Once you get to hours 3-5 then you can give full throttle, but do not accelerate from a dead stop to full speed rapidly. Gradually increase to full throttle and maintain full speed for 10 seconds max, then throttle down. During break in you never want to keep the ski maxed out for long periods of time.
Once you complete the 5 hour break in, you are good to go! Make sure you change oil and filter at recommended hourage, other than that, its pretty simple.
In regards to towing, it is technically illegal to tow on a 2up ski. It has to be 3up. I think if you have two full adults on the ski, towing your son, you should be ok. Give it a shot and see how it works. I would not recommend towing until after the recommended break in. You do not want to add stress to the ski/engine during break in.
Lastly, enjoy your new skis!
I love mine, and do not regret my purchase one bit! You will love yours as well!