If you are stretching your money to be able to afford a Hasselblad system, I would say wait until you are more comfortable with your decision. Ebay is the _last_ place I would go to buy a system, unless you plan (financially) for having the equipment CLA'd by a trusted professional in addition to the purchase price. This is true for the backs and lenses as well as the bodies.
You _could_ learn to be quick enough with a Hasselblad to take fast candid photos, but then again you may never develop that kind of proficiency with it. We all have different talents :-)
IMHO, learning enough about light, film, and equipment to be able to properly expose without a meter would be one of the best things you could do for your photography (this includes flash use). That way TTL (or an expensive meter) would be a luxury rather than a necessity.
BTW: why a 503 and not an older 500 to start with? Unless you really want the advanced features, you can save some money by buying older bodies and lenses as long as they are in good condition.