Pi,
You dont need to convert your licence to a full FAA PPL, you can get one issued on the back of your UK PPL, which you must carry at all times with you (and photo ID dont forget!), the make the FAA issued licence valid. You used to be able to walk into the the local FSDO and get one issued over the counter, FREE, within about 10mins after filling in a form, although since 11/9 you now have to make an appointment - apparently at Longbeach people were being met at the door and if you hadnot made an appointment they would not even come out and speak to you! The licence issued there and then is a 3 month temporary licence, you permanent one will be sent to your home address within 6 weeks.
As for hour building I used Rainbow Air Academy at LGB, been several times. Good choice of aircraft, well maintained, friendly staff, prices about $50-$90/Hr for SEPs and $160/Hr for a twin (off the top of my head!)
<a href="http://www.rainbowair.com" target="_blank">www.rainbowair.com</a>
Julian.