SUZUKA, Japan (REUTERS) - Nico Huelkenberg and Felipe Massa are the front runners to replace Kimi Raikkonen at Lotus next season but the team must first secure fresh investment, principal Eric Boullier said on Thursday.

"Huelkenberg and Massa are the two drivers you have to consider because of their experience and profile," the Frenchman told Reuters at the Japanese Grand Prix.

Germany's Huelkenberg, currently with Swiss-based Sauber and on his third team in three seasons in the sport, told reporters that he wanted his future sorted out soon.

"What I have basically said is that I would like to have clarification or certainty by the end of October," he said.

Boullier hoped that would be possible but it depended on the team, who won titles as Benetton and Renault and are currently fourth overall in the championship, finalising a long-awaited deal with new partners.

Lotus announced in June that an Infinity Racing consortium of private investors, including an American hedge fund manager and an Abu Dhabi-based multinational business group, had acquired a 35 per cent stake.

The team subsequently recognised the deal had yet to be finalised.

"We want to now close this Infinity partner story because this should give us the ability to be at least secure and going into the next step," said Boullier.

"There is this part and also a big sponsorship part which is being discussed and I think we are nearly there. Once we have done this we can afford to pick the driver we want.

"It could be tomorrow, it could be next week but...I will not bring Nico here if we see we cannot afford him."

Lotus are the only team yet to confirm their engine supplier for 2014, although it is sure to be Renault, and Boullier indicated this was because they were seeking a "bigger discussion" with the manufacturer.

"We would like to discuss a branding opportunity with them and maybe something even closer," he said.

"Everything is ready to be signed (for the engine), everything is the same as Red Bull (who also use Renault engines). I am just waiting a little bit to ensure no stone is left unturned."

Asked whether Huelkenberg was the main candidate to replace Raikkonen, the 2007 world champion who is returning to Ferrari in place of Brazilian Felipe Massa, the Frenchman said: "We cannot only rely on Nico, but Nico is obviously one of the targets, yes."

Massa has 11 race wins for Ferrari although none since 2008 and has said he is optimistic about staying in the sport

Huelkenberg's position at financially-struggling Sauber is uncertain, with that team seeking to bring in 18-year-old Russian Sergey Sirotkin as part of a deal with new Russian partners.

Sauber's other driver, Mexican Esteban Gutierrez brings important sponsorship.

An alternative to Huelkenberg and Massa would be a "pay driver" bringing finance with him but Boullier said Lotus, who have struggled to pay Raikkonen his wages this season, would be reluctant to venture down that road.

"This is not the way to go. We are here to compete at the highest level and the ambition of the team and owners is to compete at the highest level," he said.

Boullier said everyone at the Lotus factory had been paid and was not worried about other staff following Raikkonen out of the door. Technical director James Allison and head of aerodynamics Dirk de Beer have already left.

"Some people get nervous so obviously some CVs are circulating, I'm sure, in the paddock. But at the same time I keep them informed nearly every week, I speak to everyone in the factory, they know exactly where we are and they know I'm not lying," he said.

"I'm waiting for facts now to go to the next step and make the big announcement."