If you go 2 way, then you will need an adapter for the 6.5" speaker and will need to remove the original tweeter and fasten the new tweeter in using your preferred method. If you decide on 3 ways, then you will also need some sort of mounting adapter for the mids, I use mdf rings.
If you go 3 way, you will end up with quite a bit of bass, for me that is sufficient and gives a rather well balanced sound rather than going the sub route. Depending on your choice of sub and the setup, some subs can be overpowering.
Your best option as a stepping stone to better sound is to upgrade the fronts and amp them, if you can stretch to 3 ways, even better.