1. go to the tire manufacturer's website, look up the model and size, and it will tell you the range of wheel widths that tire will fit. normally, the stock width is probably 6" or less, but if you are able to go with a wider rim, you will need to get an appropriate offset. if not going wider, get a rim offset closest to stock if possible.

2. if you go with 16s, the aspect ratio/tire height/sidewall will not be that much smaller so the ride should be similar. since there is less sidewall, there will be less flex so handling should seem to be improved. of course if you get higher speed rated tires, the compound is usually better (ie, stickier) so handling will be better.

3. cost of wheels + the cost of removing the tires off the stock wheels + remounting them on the new wires + moun/balance/valve stems on either wheel or both