Just some stuff I have come across in my own research.
The smaller the boat you are in the bigger the waves will be compared to your boat.
Unless you plan really well and have some luck on your side you probably will encounter some rough seas along the way. You might be in trouble if you see 30 foot waves or greater if you manage to get near a storm.
I had a thread asking what size gave an ok saftey factor and it seemed to be atleast 27 feet with a 100 degree or more no recovery tip angle.
Fin keels / spade rudders can get ripped off if you hit something too so that is something to consider especially with the whole shipping container worry.
Another thing is water and fuel capacity, you will have to motor atleast a few times and a de salinator might not keep up with your water demand with the small panels you will fit on the boat.
The tank will have to take up the slack but small boats usually do not come with more than 15 gallons of water and if you are lucky the same amount for fuel.
You may be going a few weeks at a time without seeing land so you have to factor in if you can survive semi comfortably food and water wise for that time.
Someone gave me a really handy website for smaller blue water capable craft:
If you go smaller you may be comprising safety and comfort in the name of cutting cost.