Stepping up to a giant is a big step. Handling an adult giant is nothing like handling a 5 foot snake. Even a 10 ft 'tic or burm may only be twice as long but will be exponetially larger in weight. Right now I have a 9ft dwarf albino retic who is about 18 pounds but when you hold him he seems to be about 50 with him tugging and trying to explore everywhere.
Maintainence is also a lot more work. First you have to deal with putting your snake somewhere, hopefully you have an extra holding cage. Then cleaning up after them is a whole different story! Big snakes means big mess, especially with my 'tic who seems to make a mess out of his cage EVERY time.
You're going to have to consider where you are getting your feeders, local or will shipping be required? Shipping large feeders is usually pretty expensive along with the initial cost of the actual feeders. Also with a big annie you'll probably need a seperate freezer to keep the feeders unless you have a good amount of extra room in the one you currently have.
As far as an enclosure either building one or converting an extra room will be best. Unless you have the extra cash to buy one but why not try and save some cash? Boamaster I know makes a big 10 footer which would be good for most giants, though a female green annie, and possibly a huge female 'tic would be best in an extra room. The initial enclosure will be expensive along with a larger electric bill since you have to heat a huge amount of cubic space.
Then you have to consider your safety when feeding, handling, cleaning etc. You can't handle a 18' annie, tic or burm alone, 10 ft is getting up there where having two people would be nice. Do you have anyone willing to help you with moving your snake to another enclosure if need be?
These are all big considerations to take in and the choice to get a giant should not be taken lightly. The irresponsibility of so many others taking such a giant lightly is what has put our community in the situation it is in. Do as much research as you can, try finding someone local who could let you handle their giant.