If anyone has had experience in this I'd like to know how difficult it is
to
get parts for motors of this age and where do you get books or instruction
manuals for such a job. BRBR

It depends on your experience with motors.
If you work on motors, there are books available.
I got one for under 30 hp outboard motors from Intertec publishing, I forget
where on the web I found the book, but did get it. However, though not to be
as discourageing as the last poster to your question, it is not as simple as
"just follow the directions" you need to have mechanical ability or you hoped
for engine may just be an expensive basket case. But it can be done. I am not
professionaly trained, but do much of my own car repairs and have a good tool
collection. My brother and I redid the lower end on a 50 hp Merc from the
early 60's a number of years ago, even taking the shaft to a machine shop to
have it built up and lathed down in a corroded section in the water pump area.
But if you are not familiar around wrenches and seals and other motor skills,
then buy rebuilt by a qualified shop. Don't make this your learning
expecience. Just moving a 150 hp outboard is a big job, taking it apart to
rebuild it, is a BIG job. However if you can get a older, little used motor,
you may get by with just replaceing the water pump impeller and the sparkplugs
and wires. But you can easily get in over your head if you don't know what you
are doing.