First the basic questions you asked...

1: Average slip rental costs are dependent on location... in Clear Lake Texas area they run about 8 dollars a foot per month. I pay 300 per month for a 40' slip. There is usually a 50 dollar or so live aboard fee, and you generally pay your electricity bill as an added cost onto the slip rental. I pay 320 a month normally, without the live aboard fee.

2: Sailing anywhere you have to abide by USCG Rules of the Road. I suggest you purchase a copy from a local marine store or online from the USCG website... might take some searching but a google search on "USCG Rules of the Road" should come up with some purchase links.

In addition there are overboard discharge rules, too complicated to get into but there are placards at any marines store meant to mount on your boat to remind you.... it's basically how far offshore you have to be to discharge things like food, human waste, trash, etc...

3: To enter any country you need a passport, and you must physically stop in one of the countries "Port of Entry"'s You can find out whether or not a port is a "Port of Entry" in the USCG publication "Sailing Directions" which is a series of books which detail needed information about any port in the world... at the Port of Entry you will need to check in with Customs, the process differs in each port, and in some ports fees or inspections are overlooked if you discreetly offer "gifts" of say coffee, or liqueur, or cigarettes.

4: It's not terribly difficult to single hand a sailboat so long as it's set up so that the rigging runs to the cockpit... however... I would not recommend doing long trips solo as even in the open ocean you can run afoul of traffic from cargo ships who cannot see you... keeping someone on lookout at all times especially at night is absolutely important. You should have no problem with your son as a deckhand once he learns what he needs to do to help you.

5: 70's era Sloop rigged sailboats like Catalina's, Rangers, etc in the 30-36 foot range are generally blue water seaworthy and can be run solo. They range from about 14k - 30k in price depending on what kind of gear they have and how good the interior is. Regardless of the boat, if you find one you are interested in, have about 500 bucks available to do a pre purchase survey. These boats should have an inboard diesel motor around 20-35hp. These are very useful for in port maneuvering which is impossible in yacht harbors without a motor. Interestingly enough, I purchased my 33' Ranger for 14k, and it's 28hp Yanmar engine is worth nearly half what I paid for the boat.

Next I have no doubt you are fully capable of living this life, just keep in mind. Learn your boat extensively, so you know how she handles in most conditions before taking long offshore trips. Stay within a few miles of land on those trips when you can till you are truly salty and know how to react to all situations.

Also, while having GPS is crucial these days, buy a Sextant and research the publications you need for Celestial Navigation and learn it and teach it to your son. Civilian GPS can be turned off at a moment's notice from the Pentagon (the whole system is owned and maintained by the US Navy)... and would get shut down in any war with a technologically advanced enemy.

Also Investing in a Navigation program like Capt. Navigator and electronic Charts is something I highly recommend in addition to full size paper charts of the Coasts you are traveling.

There are one week courses all over the place that teach Celestial Navigation and range from 250 in state at maritime based community colleges, to 2000 dollars at privately run Maritime schools. (Celestial Navigation is still required for licensed mariners who sail the oceans professionally.)

Also spend some money or find someone who will take you and your son out sailing on the size vessel you are thinking of buying... learn the basics if you don't already know... and learn any tips/tricks they might offer in addition... all sailors have some tip or trick they ran across somewhere to make life easier.

And finally I would suggest going through a USCG approved STCW and Able Bodied Seaman course... even if you aren't going to be a licensed mariner, those two courses (a week long each) offer invaluable knowledge on seamanship and safety that can and most likely will save your life or your son's.

So...

USCG Rules of the Road
USCG Sailing Directions for the areas you wish to go.
Electronic Chart/Nav Program
Sextant and Celestial Nav Publications.
USCG approved Celestial Navigation course
USCG approved STCW course
USCG approved Able Bodied Seaman course

There are supposedly yacht safety courses, but I've never seen any that hold a candle to Merchant Marine required Courses I suggested above.