As far as I know, the over the counter 500mg dose is sold over here as a painkiller, it's certainly not meant for long term preventative use.
Most def', but if you ask for aspirin without specifying any further that's what you get (that's what I'm given every time anyway). I'm not sure you can even get 75~80mg, 100mg seems to be the lowest dose available (for long-term use after infarction & stuff, and for toddlers).
Im glad my dr is a sane one. I dont see anyone needing high doses unless your dealing with a mayocardial infarction where high doses of aspirin are administired to prevent damage to the heart.