|The shop manual describes the stock wheel as being cast aluminum with a flat hump and a safety ledge.
Numerically the wheel is 5.5J-13" with an offset of 0.75" giving a backspace of 3.5" (to make this absolutely clear, the car side of the wheel is noticeably deeper than the street side). Note that the backspace measurement follows the British convention which IIRC is measured at the tire bead mounting surface, not the US convention which uses the innermost surface of the wheel.
The stock wheel mounts via four 7/16" diameter studs on 4.00" centers, not 100mm which won't fit. You'll probably want your wheels machined for conventional flanged mag wheel lug nuts rather than the stock lug nuts, which are unique to the JH. Installation torque is 51 ft-lb dry regardless of lug nut type -- and since replacement studs are $5-$6 EACH you will not want to exceed this limit.
The center hole is 2+3/8" diameter but if necessary you should be able to go to 2" or even 1+3/4" before the wheel would interfere with the front hub. If you're planning on using aftermarket hub caps, make sure they, too, will fit over the hubs.
The inner diameter of the rim is about 12.5" but this tapers considerably so you'd need to check each individual wheel style on the car to see if there were any clearance issues.
As an alternative, you might wish to consider having the stock wheels abrasive blasted and painted or polished. You can get some very stunning looking wheels that way, at a much lower cost than for new ones. Of course they're still 13" wheels....