Unfortunately, there is no such thing as a fast, comfortable, lightweight tourer. Tourers are known to be heavy and slow, but if set-up correctly, very comfortable. Taking your time and enjoying the scenery is what it's all about.
Going second hand can get you a good bike for alot less money, but finding what you want in the size you want could prove to be difficult.
The only tourer that I personally know of that would fit into your wish list would be the Dawes Vantage and goes for around £600 new. My recommendation on this bike is biased as I have one and I love it to bits. The other reason I advise it is the sizes it's available in. The smallest is 17". I'm 5'3" and the 19" fits me quite well, but with a lady's shorter torso, the 17" would be better for you. Going to a bike shop and trying it out for size is the best option though.
Another bike I would suggest is the Specialized Tri-Cross. It isn't a tourer as such, but a general purpose do everything bike. Lighter than a tourer, but comes without mudguards or rack and goes for around £750.
One other option, is to just keep your current bike, fit mudguards, rack and panniers for when touring and bobs your auntie.