Let's be clear about one thing: Sébastien Loeb Racing isn't quite the same thing as Sébastien Loeb racing. The man is a 9-time rally champion who's dabbled in other motorsports; SLR is a team founded by Loeb and partner Dominque Heintz, which fields several cars driven by racers other than Loeb: an Audi R8 LMS Ultra in FIA GT3, a 911 GT3 Cup in Porsche Cup racing, and an Oreca LMP2 car which competes in European and Asian Le Mans Series events. And so when we say that (the name for the partnership between SLR and two other companies) is developing a new LMP3 car, it's not that Loeb himself will be building or driving it.
That being said, the new car is interesting in that it's developed for the new ACO LMP3 class, which as you might be able to gather will slot "below" LMP2, a cost-controlled entry-level prototype packing a Nissan V8 making 420 hp and weighing around 1900 lbs. SLR's partner ADESS AG will design and homologate the car, SIRA Racing will produce the body and chassis, and SLR will field the car. Much like LMP2, the ACO and SLR expect that the LMP3 cars will be a stepping stone for younger drivers to advance as well as a way for the ubiquitous "gentlemen racers" to give LMP a shot. It'll also be relatively cheap; the ACO expects the cars will cost in the range of $240,000 US and cost less than $500,000 US per year to race.
There will also be a "Challenge" version sold to customers for use in some non-professional open sports car series like the Excool OSS Championship in Britain. That variant will make 500 hp.