Here are your answers in full (but shortened thanks Ally)
1. The RB5 stands for Richard Burns and YES his car was number 5. It commemorated the return of Richard Burns to the Rally and camp!!
There was only 444 RB5 produced and they cost new £24,995 when first built in March 99, However prodrive did a performance version of the RB 5 and it had better engine response with a 238 power output!
2. The terzo was the 'third' special edition made in the UK in april 1998. 'Terzo' actually means third in Italian and it was called this to celebrate the Impreza's third succesful victory in the WRC manufacturers title. There were 333 Terzo's made however if you live in Switzerland the terzo was marketed and called the "collection".
3. The STI type RA means Rally Applicable thats why they came with the roof mounted vents. They were also made to be very lightweight for Rally purposes. And only came in white!!!
Wullie