For billionaire investor Nicolas Berggruen, signing a deal to acquire insolvent German retailer Karstadt Friday was only half the battle.

"From the standpoint of a pure acquisition, this was probably the most challenging deal I've ever done," the seasoned investor said in an interview Saturday with Dow Jones Newswires. "But now the hard work really starts."

Berggruen has just acquired one of Germany's most storied and troubled retail brands, a sprawling department store chain with 26,000 employees and around 120 stores. Already pressured by high rents and a drop in business, its woes intensified last year when parent company Arcandor AG became insolvent, which led to the search for a buyer for Karstadt.

Berggruen says his first order of business is to revitalize Karstadt's sometimes dowdy mechandise, which has turned off younger and trend-conscious shoppers in recent years.

"Its going to be more dynamic. Not only more fashionable but also faster in terms of changing more quickly with the seasons," Berggruen said.

Berggruen says he's already in talks with a number of potential retail partners interested in offering or expanding their product lines at Karstadt. A previously announced partnership with U.S. fashion label BCBG Max Azria - part of Berggruen's efforts to revitalize Karstadt's look - is also planned.

New additions to Karstadt's management are also in the works, although Berggruen says he himself will steer clear of the company's day-to-day operations.

"You need new blood and fresh ideas," Berggruen said, although "I don't operate any of the businesses that I acquire."

Many of Berggruen's other investments focus on sustainability, such as renewable energy projects in Turkey and agriculture. Other investments include real estate, hotels and a car service in India. His holding company Berggruen Holdings has more than EUR2 billion in assets under management, according to its Web site.

Retail industry experts have for years mulled the possibility that Karstadt and its national rival, Metro AG's (MEO.XE) Kaufhof, could merge to create one giant German department store chain.

Such a tie-up looks unlikely for the time being under Berggruen's ownership, however.

Berggruen says he remained committed to getting the company back on its feet as a long-term investment and "doing what is best for Karstadt."

He is also confident that his relationship with Karstadt's creditors won't stand in the way of reviving the company, even if several months of heated negotiations with creditors resulted in strained feelings.

"We have a very clear situation. I'm not dependent on them. In fact, it's almost the other way around given that I'm the one paying the rent," Berggruen said. "This business has no financial debts, even if some banks have a different opinion."

Berggruen won a tentative bid to acquire Karstadt in June dependent on reaching an agreement with property owners and other creditors about lowering store rents and other lease-related matters. Several creditors including Highstreet, a real estate consortium led by Goldman Sachs Group Inc. (GS) and the owner of the bulk of Karstadt properties, fiercely contested the takeover plan at first. Karstadt's insolvency administrator planned to shutter the stores if a deal wasn't finalized by the parties by a Friday deadline.

In a surprise shift in sentiment, Berggruen lauded Deutsche Bank AG (DB) and its head of German operations, Juergen Fitschen, for acting as a mediator in the negotiations process with Karstadt creditors. Berggruen had previously accused Deutsche Bank, a member of the Highstreet consortium, of "playing with fire" for its last-minute meddling in the negotiations process. But Fitschen "made an effort to get the thing done," Berggruen said.

And rare for a takeover artist, Berggruen credited Karstadt's union and the German government for preserving the deal from failure. "We got a lot of help from people who didn't want Karstadt to fall through the cracks."

Berggruen wasn't shy, however, in venting his frustration about the prolonged negotiations process.

"There was one roadblock after the other," Berggruen said. A student of existentialism, he compared the takeover process to Samuel Beckett's absurdist play "Waiting For Godot," the story of two men waiting for a man who never shows.

But unlike in Beckett's world, Berggruen's deal finally proved a success with an Essen court's stamp of approval granted Friday.

Berggruen is sometimes called the "homeless millionaire" because he doesn't own a car or home, instead travelling the world in a private jet in search of new investments, charitable ventures and high-profile social events.

Having completed negotiations for the Karstadt deal in Berlin, Berggruen said he's next headed to Madrid where he will work on his Spanish media company, Prisa Group, and to meet with a member from the Berlin-based political think tank he has founded.

Looking further into the future, Berggruen says he's considering an investment in a Europe-wide real estate business, more media companies and a U.S. sector that's "too controversial" to talk about just yet.

By William Launder, Dow Jones Newswires; +49(0)6929725515; william.launder@dowjones.com