Overview

Mid-engined cars like the Superlite SL-C typically use a transaxle, instead of a separate transmission, driveshaft and rear end.  This simpler configuration also has intrinsically less drivetrain loss compared to a conventional transmission/driveshaft and rear end.  It also has packaging advantages.  However, transaxles also tend to cost more, and are somewhat more difficult to find. That's why the factory designed to SL-C to be able to use a wide variety of transaxles, across a very wide range of cost and performance. 

Porsche G50  Available from recyclers for as little as $4500, cleaned and setup for inversion in a mid-engined car. These are good for cars with up to 450 ft/lbs of torque, depending on their condition. A Porsche G50/52 from the 89 turbo, is stronger, and will accept more power.
Porsche G96   The G96 series was available on the 996 series of 911s.  These are easily available from wrecking yards, and are also available from some sources as remanufactured transaxles direct from Porsche, presumably with a warranty, for under $5000.  These are a value choice for the SL-C, as they are relatively robust, inexpensive, have later technology, and share the same basic architecture as the latest street transaxles. 
Porsche Cayman  New transaxles from the Cayman line that are available for around $7500. These are a great solution for engines with under 500 HP/500 TQ. 
Porsche G97

The new G97 transaxles from the late Porsche turbos are also great transaxles, and have withstood 600 HP in the factory race SL-C, on the way to winning the 2011 NASA National Championship.

Ricardo Transaxle from the 2005-06 Ford GT cars are extremely robust, and routinely power 1000 HP cars. These are extremely expensive, hard to find, and the clutch and starter are costly as well, but they are bulletproof.
Graziano From the same maker that provides transaxles to Audi and Lamborghini, these brand new transaxles offer OEM reliability, availability of new units at the price of some used units.  These are 6-speed units, with high-end parts.
Mendeola Mostly an off-road supplier, now entering the street market. 
Griffin  Transaxle under development. 
Others  The SL-C will also accept racing transaxles, as from Sadev, EMCO or XTRAC. These are very expensive and have frequent rebuild cycles, but are designed for the rigors of racing, are usually sequential shift, and are relatively light. These are really for racing purposes only. For 2013, the factory SL-C will be running a new transaxle from Sadev, developing it as a solution for those owners who want lightning-fast shifts just like the prototypes in the ALMS and GrandAM.

 Most of the existing SL-Cs are using the Porsche or the Ricardo transaxles, but others will fit as well. For example, transaxles from Mendeola fit the SL-C, as will the ZF that is popular in the GT40 and Pantera, and RBT-sourced version of this transaxle. 

Most of the current SL-Cs are being supplied with the Graziano transaxle.

Gearing

The right gear ratios often make the difference between an enjoyable drive, and a buzzy one.  Or for track rats, the difference between having the right gearing for the track- or not.  The link below, courtesy of LXM, is a very useful spreadsheet that has ratios for popular Porsche gearboxes, as well as the Ricardo and new Graziano gearboxes.

Click here to download the gearing spreadsheet.

Shift Cables and Knobs

The kit ships with a shifter from Brandwood, and typically, with cables for your transaxle choice.  Depending on how you route the cables, you may elect to order new cables of a different length (at your expense, of course).  These can be sourced from Brandwood directly- they know what most of the transaxles need, and can provide custom-length cables as needed.

The shift knob pitch for the standard Brandwood shifter is 1/2-20.  For the original Ford GT shifter used with some of the Ricardo transaxles, the pitch is M12 X 1.75.

Speed Sensors

The transaxle is a great place to put a speed sensor, if you need one. For many LS engine tunes, the ECU needs a speed sensor input to help control idling.

Since virtually all SL-Cs have Porsche 930 CV joints (they are supplied on all standard axle sets no matter what transaxle is used), a ring around the outside of the joint is a popular way to get wheel speed.

The supplier of the axles for the SL-C, The Driveshaft Shop, has a ring that fits around the outside of the CV joint. Their website doesn't show it as a part, but if you call and ask for it, they have them.

Another supplier of these is LatestRage. Theirs is a bolt-on as opposed to the slip-on style provided by DSS.

If you need a factory-style VSS (Vehicle Speed Sensor), here's a link to one that works on the LS3 ECU, and presumably the others as well.