Recommended tire pressures are on your driver's side door jamb.
For my E90 xi it is F:32psi, R:36psi
Springs and shocks will definitely help with the body roll, removing most of the feeling. For a daily driver, I think this will be enough.
One bit of information I have picked up about shocks is that Koni FSDs tend to over/under-react to different driving conditions. I've heard the Koni yellows are much better than the FSDs.
Coilovers are nice for track setups and are usually overkill for the street. However they do have their advantages. You obviously bought an xi so you can drive it in the snow. The reason you get the floaty feeling is because of the higher suspension...there are a number of other things that contribute to it as well.
With coilovers you can lower your car in the spring/summer/fall and raise it for winter without swapping the whole suspension. Obviously you can't do this with a shock/spring setup.
It really boils down to what you intend on using your car for....