Advice

Pay Attention To The Bolt Pattern
Make sure the bolts you have do fit the newly acquired wheels. Bolts may differ for different types of rims.
If the bolts differ, the best case scenario is that the bolt heads are longer and they stick out, making it impossible to put on the safety cover.
In the worst case scenario, the bolts have different threads, and you won't be able to mount your new wheels.
Even if you change your rims it is better to keep at least one set of the original bolts so you can use the spare in case of a flat tire.

Bigger Wheels
The bigger and wider, the better looking they are.
If you want to fit bigger wheels on your car you'll have to get lower profile tires so the total diameter of the wheel will be the same.
Low profile tires will give you better cornering and lateral stability.