Since this thread is so big, here are Cliff notes.
Here are the part number from from Bilstein
The rears are from a C4 Corvette and the fronts from a Nissan Frontier. The rear shocks should be very close, but the fronts may have too much rebound.
You can enter these part numbers into Ebay and find them for very good prices.
To use these as coil overs you need coilover sleeve kits from Allstar Performance or other seller on Ebay. Most of these kits require machining a groove into the shock body for a circlip that holds the threaded sleeve. You will also need springs. 2.5" ID springs are needed for front and 3"ID rear, which can be found in any spring rate on Amazon or Ebay for cheap. Not sure of the spring length needed and will try to get that info from Bilstein.
It is probably easier to get Gaz adjustable height shocks, version GP8-2144 front & GP8-2145 rear
(ask for large 3" iD spring perches). Beware: 2.5" coil overs will NOT fit the upper rr mount.
The spring rate/length/damper question is more complex than it might seem because Lotus was always fiddling with it to make performance improvements, meet ride height regulations, and adjust oversteer/understeer characteristics.
Here are a few specific model specs that might help.
Lotus was a pioneer in Big Bar Soft Spring Setup (BBSS) application, google it.
Thus, arbitrary spring rate changes don't work well. Don't adapt S4s rates to G-car, your chassis will develop cracks.
Non-SE 89 including G-cars:
Front: Rate – 22.8 N/mm / 130 Lb/in, Free Length – 369mm, Static Length – 203mm
Rear: Rate – 27.5 N/mm / 157 Lb/in, Free Length – 334mm, Static Length – 205.7mm
Front: Rate – 29.1 N/mm /166 Lb/in, Free Length – 372mm, Static Length – 219mm
Rear: Rate – 27.5 N/mm / 157 Lb/in, Free Length – 347mm, Static Length – 218mm
The front damper platform height for each of these measurements is 59mm. The rear platform height is 115mm.
The extended/compressed length for the front is 12in./10in., and the rear is 16in./12in. If you increase the spring rate, you have to shorten the spring to achieve similar preload (vehicle ride height).
S4/S4s -those had STIFFER/STRONGER Chassis
Front: Rate – 41 N/mm / 234 Lb/in, Free Length - N/A, Static Length – 214mm
Rear: Rate – 45 N/mm / 257 Lb/in, Free Length - N/A, Static Length – 227mm
Ride quality is a function of a fine balance between understeer and oversteer and does not respond well to random changes.
It is all out there:
For more info see Post #86 and on, 99, 130