Wow... I really appreciate all the replies, suggestions, and insight. I have 2 young boys and whatever I buy, I plan on keeping for a long time and possibly restoring it with the help of my sons. That is my plan anyways.... Their plan may differ!

This is specifically why I ask about parts availability. I live in an area of Michigan that is rural and has virtually every dealer for every manufacturer within a half hour or less drive.
The more I read up, the more I am learning. I guess every 65-80's vintage tractor has their issues. So maybe some of you can clear the air about a couple things I have read about the brands that have been discussed in this thread.
Ford 3000/4000 3 cylinders - Not many bad things. Good all around tractors. I have read that the engine blocks are porous and may be susceptible to cracking? Also, that engine rebuilds can be very expensive. Any other known issues with these?
IH 484/584/574 etc - Parking brake issues, unbearable heat while operating during the summer months, dry clutch issues, and problems starting in cold weather? Also make better loader tractors than field tractors?
Massey 150/165/255 - Weak Hydraulics, bullet proof Perkins, run far away from multipower if it isn't working 100%. I heard this can be a very costly fix. Many more available 500 miles south of me.
Should I be looking at anything else? The Oliver 1550's are super cool. I really like them... But there doesn't seem to be many out there and I wonder about parts being hard to find.
I was given a local referral to a guy named Dan Gingell and his daughter Rachel who seem to be very popular in the vintage tractor scene.. They live 2 roads over from me. I may give him a call and pick brain or better yet, maybe he knows of something local that I would be interested in.