As with any performance car, stay away from modified cars even more so if its your first time buying one. While the SR20DET engine is bulletproof, it is not without fault and a car modified the wrong way, and then handed down to a first time owner can ruin the entire experience for you, even more so when its an S15.
For daily driving, you might want to consider one that still has the stock suspension...Nissan engineer good stuff from the factory, and unless you're living where i do, they will hold up and will give you a good feel of how it is suppose to handle rather than buy one with an aftermarket coilover that has been tuned to a mysterious spec...the chassis is capable of drag, drift, autocross, rally and Vmax so getting in one that has the suspension tuned for something other than what you want can make you feel effy. This is my personal experience and only recently in my 6 years with my car have i found that the best compromise for day to day driving and track use is the Nismo S-Tune Suspension system.
Also have it on a lifter at a trusted garage and inspect the underbody. Make sure your frame rails are nice and straight with no flattening or distortion that may indicate an accident or a bad repair in the past. Electronics and rust are not known problems except for tiny spots on the rear subframe. As for the gearbox my car makes 370ps and 412nm and has not broken yet so it'll hold up fine. Its normal for the lever to rock gently in position or in gear, the changes should be smooth especially in 3rd (either upshift from 2nd to 3rd or downshift from 5th to 3rd ). Ive driven cars which engaged hesitantly or after hard shoving and these are the types you wanna avoid. Also note if you do intend to buy one which has a power upgrade, make sure the clutch has been upgraded to suit the power as well...And brace for the best decision you have made yet.