As far as the IMC is concerned i would say it all depends on how you intend to do your hour building.
If you are doing it in the UK over an extended period then yes, id say get an IMC and use it as well. No matter what anyone says it will make the IR easier as you will have become used to certain procedures and have less to concentrate on.
If your hour building involves going to the US and knocking out as many hours as possible in as little time as possible then dont bother. You obviously aren't interested in using the 100 hours to make you a more skilled pilot in a variety of situations as its intended to do.
Too many people just see the 100 hours PIC requirement as an obsticle to be gotten around in as quickly, easy and cheaply a way as possible. This is the time you should be mixing up your flying a bit and enjoying it too. Get as many extra skills as you can.
I cannot believe some of the posts on here saying it is a waste of time. Anything that makes you a better pilot is not a waste of time whether is aeros, floats whatever.
While were at it dont just do one long CPL flight either, get yourself used to doing long flights early on in your hour building. Get yourself down into France etc. Once you realise how easy & rewarding it is you will do it more often anyway.
There is nothing more useless than 100 hours of flying round Florida done in one month along with long periods of no flying.