Just bought a '98 sedan and the ride is brutally harsh for the roads in my area. I bottom out or hit the bump stops almost every time I drive.

It has 18" rims (235/40 tires) and aftermarket springs (don't know the brand) with stock shocks.

Will it help the ride much by putting in stock springs or am I doomed to bruised kidneys because of the 18" rims?

the M stock shocks are not designed for aftermarket springs and lowering, therefore, if you have aftermarket springs, you must have aftermarket shocks to go with it. They work together. Thats why you are bottoming out left and right. Get yourself a set of Bilstein sport shocks to avoid damage to your car. I bet you have H&R or Eibach springs. Shocks replacement is a must if you gonna change your suspension setup.