I have seen several boats with bulbs added to the keel by Mars, and they looked fine--as if the keel had been built that way. I understand that Mars does quite careful calculations to determine how much keel to remove and the weight of the bulb to add, to achieve whatever the objective is--sometimes simply to reduce draft, sometimes to stiffen the boat in stronger winds, etc.
Although I would take a very close look whenever a boat has had major modifications, I would not rule it out based on a modified keel. But I would ask for detailed information on the reason for selling the boat, reason for the keel change (for example, did the boat run aground?), the calculations involved in deciding how to change it, etc. I would also obtain an independent survey with special attention to the keel, keel bolts, any structural damage in the keel/hull joint, etc. Finally, I would take her for a test sail in several different wind conditions--both light and heavier air, to see how she performs under sail with the bulb keel.
Good luck--let us know what you find out, and what you decide.
Frank
