As far as Timothy Donahue is concerned, everything is just fine with the Hannaford grocery store he frequents in Mill Creek in South Portland. “I’m very comfortable with Hannaford. I know where everything is and I can shop quickly,” Donahue said Wednesday after learning that Hannaford’s parent company, Belgium-based Delhaize Group, is being bought out by a larger European grocery company, Ahold, which is headquartered in the Netherlands. So Donahue, who lives a few blocks from the store, had one wish for Ahold. “Hopefully, the new company is smart enough not to mess it up,” he said. Donahue’s comments illustrate the fine line the owners will have to walk as they combine the two companies, which last year had sales of more than $54 billion and will serve an estimated 50 million customers a week in the U.S.