I'm not too sure about the "I built this 20lb bike off the classifieds for £100" brigade. Maybe if you've got some good spares knocking around already, and can devote a lot of time to trawling classifieds – if something's cheap, it goes very quickly.
A mate recently bought an Orange Gringo with a variety of good to gopping kit on it. The frame, bars, shifters, BB, headset, rear mech and seatpost were useable. Everything else wasn't. He took stripped it, and I sourced and fitted all the bits to make it back into a decent bike.
I bought the vast majority of the kit off here, at pretty good prices. I did get some new kit – Shimano cranks, new chain, new cables and new grips. Everything else was second hand. The build still ended up costing £200, and I've loaned a set of spare forks & a saddle to the mate. If I'd sourced everything but the frame off here, it would have cost about £290-£300, for a build that's a touch better everywhere than what's on that Giant. As I was sourcing a lot of second hand kit, I inevitably had a couple of headaches with stuff taking a while to turn up etc.
If I were in your position, I'd pull the bike to bits, sell the frame, headset & bottom bracket on ebay (seems to get better money than here), and buy a Dirty Jo or Handsomedog frame, transferring everything over. You'll probably get £30-40 back for your frame, be able to get a new frame for less than £80, plus £20-£30 for a new headset and bottom bracket from the bike shop.
The Handsomedog / Brand-X and Dirty Jo frames are all pretty similar, and are all pretty well regarded. Cheap, but not necessarily nasty. I'm agnostic about the idea of nice bits on an OK frame or OK bits on a nice frame.
That way, you get the pain in the neck / special tools jobs done by the bike shop, and a fairly hassle free build process. As time goes by, I'd buy a set of hydraulic disk brakes (Shimano, perhaps second hand for £50) and a better fork – I've no idea how good the Suntours are.
I'd not worry about 24 vs 27 speed – Alivio kit seems to last forever. I'd leave the change to 9 speed for when you break something, or change bikes. Bits are always a lot cheaper as parts of whole bikes than on their own.