There are aftermarket cruise control kits that you can probably retrofit more easily than a factory system.

You can fit power memory seats and door panels with switches if you can find them in material and color that matches your car. The wiring and necessary modules and relays will be a difficult task.

Depending on where you are it might be cheaper to sell your car and buy one with the features you want.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD