There are a few things to consider when you're doing a coil-over conversion.
1. Shock/Struts: if you're going much stiffer than what you have right now, you better make sure that the shock or strut you are planning on using will be able to keep up. Most of the cheapo shocks are usually not very good at over 700 lbs spring (200 lbs Coil-over rate), so if you go stiffer than that on a Tokiko HP series for example, you will likely have a bouncy and uncomfortable ride.
2. Manufacturer: While all coil-over kits are equal in theory, some are more equal than others. Stuff manufactured by companies that have major profile in motorsport is usually better built, has higher quality components, and will fit better. Companies such as Agent 47, Griggs Racing, and Maximum Motorsports are just a few. Try to avoid stuff that made in China, materials are cheaper, possibility of service is unlikely, and fitment will probably suck.