Hi,
Welcome to the forums.
Firstly, with regards to buying a PJ, research. Think about what type of unit you want. DLP? LCD? Use the internet and buy this months edition of What TV and video as it has a reasonable suppliment on buying a new home cinema projector and the subsequent pitfalls or bonuses.
Having built a cinema room in the past my first piece of advice would be to plan it from start to finish. Dont worry about the size of the screen yet. Look at projectors, demo them if possible and when you find one you like work out where you place the PJ and then think about the suitbale size of the screen. When you are setting up the room a 6 foot wide screen is big but you could get away with a 7 or even 8 foot screen in there.............
Think about wiring where is going to go etc.
Hope this helps. With regards to packages there are some fantastic companies around that you can use. Check out -
www.discounttv.co.uk www.nexnix.co.uk www.intoav.com
These are all superb UK companies with excellent customer service who may be able to advise you on what you need.
Cheers.
Cap