In battle square when you've bought the omnislash and w summon and you get to fight the 'special' monsters, how much HP does the last guy have?
120,000

is that 'championship belt' worth buying? what does it do?
Yes. Strength and Vitality + 30

where do you actually win them from? betting or riding?
Racing.

Also, is there a sort of 'trick' to getting the right item?
No.

Where is the w magic materia?
Here. Third screen of the left-upper route in the Northern Cave.

Quote Originally Posted by VeloZer0 View Post
1) My sources in the EoFF FF7 page tell me 60,000 HP.
That's how much Proud Clod has when you fight it in Midgar. All enemies in the Battle Square have 100% more HP and 25% more Attack and Magic.