Before you buy anything else, I recommend you buy this book:
The long Distance Cyclists' Handbook by Simon Doughty; it explains, layman's terms, all aspects of long distance riding and also includes training programmes for various distances. A small investment now, will repay you many times over in the future.
I don't doubt your friend is a serious cyclist and knows his stuff, but I would guess that his area of expertise isn't long distance riding: 99.999% of Audax and touring cyclists (statistic made-up for emphasis) ride bikes with steel frames and leather saddles, neither of which are the lightest available, but if you are spending hours in the saddle they are the most comfortable.
I think you are right that upgrading your current bike is an expensive way of going about things; even then you will only have an
adapted bike, rather than a
purpose-built one. Either way, the frame is Aluminium which is not a good starting point for a long distance bike.
By all means replace parts with something more suitable to your riding as the wear out (wider tyres, a wider and higher range cassette etc), but your money is better saved for a purpose-built bike: sow's ears and silk purses etc.
With £600 I would be looking to buy second-hand: a new bike for that money will likely need the wheels and tyres upgrading, where as a second bike bought from an enthusiast will have had all the tweaks made. I suggest you join the
CTC and
YACF forums: both have an emphasis on distance riding, so lots of advice from experienced riders and your best bet for finding a suitable, well maintained and good value bike.