If you are using the Speed P8, you don't have to get another crank set, just buy another 39 teeth chainring and bolt onto the existing one. That will save you a few hundred bucks. The complete parts list is the RD, FD, FD adapter, and the set of shifters. If you buy 8 speed shifters, it is hard to get quality ones as nowadays 8 speed is only used on cheaper bikes. Go for a 9 speed if you want to save some money and Alivio or Accera are decent components.

If you are using 406 wheelsets (20 inch) , I think the biggest cassette is 32 teeth and maybe 34 teeth if you shorten the chain a bit. This will pull the cage to the front rather than position itself vertically when on 34 teeth setting. It will also be very near the rims and if you are using Big Apples, the cage will be rubbing the tires on this setting. So, recommended to use 1.6 maximum tire width. If you can afford go for 10 speed off course. The Tiagra components should be OK and get the original Tiagra 12/30 cassette should be sufficient even for Fraser's Hill ride.

Happy Modding. 

If you decide to use 8 speed, any RD capable of 8.9 or 10 speed can be used. The shifter determines the distance for the RD cage travel, not the RD itself. So, 8,9 and 10 speed is within the 8 speed's travel. There are the upper and lower limiting adjustments to adjust the cassette's distance window. Also, 8 speed means the maximum size cassette is 11/32 but there are smaller ones and for 32 , you will need long cage. If you choose 28 or 30 as in road bike cassettes, a medium or short cage  (road bike RDs) is sufficient.