They were using the 2047's with most engines. The 2062 has been designed for slightly more bottom end than the 2047. Ive been running a Hong Nor 053 (which is not race legal, due to it being a 2 chamber) with my 21-5KT I will be trying a 2047 on mine next weekend and will tell you how it goes (I am doing truggys at the moment though not buggy's)
Depending on if you prefer more torque or more top end go with the 21-5KT or the 21-7KT and then the same with the pipe. I would go with the 21-5KT and the 2047, start with 14 tooth clutch bell (most buggy's come with a 14 as stock) with the 5 port you'll get more fuel economy and you cant use the GO .25 in a buggy
The BRCA Hyper 8 is very good to start of with though, and during the year you can upgrade the radio gear and engine etc... but it does normally come out more expensive
Have a look at the SMD power packs as well, there a great deal and I've had great luck with my 7 and 5 port,
GO 7 Port + Pi-811
GO 5 Port + Pi-811