I think, and I may be wrong, that bruising and old blood in the white line is usually from old damage higher up the hoof wall that has grown down. It certainly suggests your horse is prone to problems, and her previous lifestyle sounds like it won't have helped either. The stretched white line is also a sign of grumbling problems, along with flare in the walls (ie bell shaped rather than straight).
Look out for heat in the coronet band area and raised digital pulses (feel for a pulse in the crease below the fetlock joint) as they are the usual signs of trouble, although they don't automatically lead to a full on attack.
Also consider what your are feeding - a lot of conditioning mixes contain high levels of starch and sugar which are also problematic, if you need to keep condition on quality chaff and oil are a better way to go.