I was going back over your original post and noticed that your room is about 4M (13ft) x 4.5M (15ft). That is a little small.
I was originally going to push for the Dali Concept 8, and while they would work, these are some monster sized speaker, and might seem out of proportion, though you can use your judgement.
However, the Dali Concept 6 would be fine (£390/pr).
As to the suggestion of the Acoustic Energy Neo 3, from everything I've heard, people really like this speaker. Well below your budget, and an exceptional bargain at under £300. Certainly worth giving a listen to, and would work well in your room.
I think in new speakers, the Wharfedale Diamonds are also worth a look. They have many sizes to choose from, from the single 6.5" 3-way 10.5, to the twin 6.5" woofer 2.5-way 10.6, to the larger twin 6.5" woofer 3-way 10.7.
As to the used 'Alpha' amps, do it if you like, but I still say you will not come up with better quality or value than the NAD C355 for the money. Plus, you get full manufacturer's warranty, that's worth something.
The only thing the NAD amps don't have is a turntable input, but it seems as if that is not a problem for you.
In bookshelf speakers, this gets trickier. You are not going to get the same bass from a bookshelf speaker, though they can still be very good and very satisfying for stereo if you pick wisely.
The Wharfedale Diamond 10.1 with a 5" woofer, has gotten some extremely positive reviews since its release; always a good bet, but a little small for my taste. £169/pr from several sellers including SuperFi.co.uk .
Better would be the Diamond 10.2 with a 6.5" woofer. A 6.5" woofer is nearly 70% bigger than a 5" woofer. Keep that in mind. Low end response is rate down at 40hz, which is very respectable for a bookshelf speaker.
Superfi.co.uk is the first UK seller to start offering discounts on the new Diamond 10 series, and I think soon other sellers will follow their lead.
The Monitor Audio RX1 and RX2 are in your budget range.
The RX1 has a 6" bass driver and response down to 45hz; price £400.
The RX2 has an 8" woofer and response down to a very respectable 40hz; price £500. This is not a pixie speaker, but I would expect it to sound very very good, a deliver weight well above the typical bookshelf.
The Monitor Audio Silver RX series are very highly regarded speakers, and you should find several discussion of the bookshelf model in this forum.
Based on reputation, for clarity, consider the Dali Lektor line of stand mounted speakers.
The Dali Lektor 2 is about the same as the Diamond 10.1, both have the same low end, and both have the same sized woofer. However, the Lektor 2 is £299 from HiFix.co.uk, but it might be worth shopping around.
The Lektor 3 has a 6.5" woofer, not much more bass extension than the Lektor 2, but again, it is pushing a lot more air, and that has some value. The Lektor 3 from HiFix.co.uk is £449. It might be worth checking prices at Richer Sounds, I think they might be about £50 less.
The Dali Concept 2 is available from Audio Affair for about £250/pr, and has response down to 43hz from a single 6.5" woofer. The Dali Concept 1 is about £175/pr, 5" woofer, response down to a not so good 63hz making this useful for rear surround speakers only.
The Dali Ikon bookshelf on the other hand, I would expect to be very impressive speakers. The Ikon 1 has a tweeter/super-tweeter combination along with a 5" woofer with response down to a very impressive 43hz. Price at around £350 from Richer Sound among other places. The Ikon 2 with an equally impressive 42hz low end has a 6.5" woofer. I'm seeing these price between £670 and about £700. In this case, the best price seems to be from HiFiGear.co.uk (£671.77), but Audio Affair is running a close second at about £689/pr.
Again, from everything I've heard, the Ikons are stellar speakers. Keep in mind the prices and models are for the older standard Ikon line, which has just been replaced by the more expensive Ikon MK2 series.
I'm not intentionally pushing Dali speakers, they just happen to have models in your price range, and they have along history and sterling reputation.
Though, I have to admit, as it is with everyone, my recommendations reflect my personal biases and preferences. I can't divorce myself from that. But I think I can safely say that there isn't any bad equipment anywhere in this discussion. It is just a matter of now finding which you prefer.
Most of my prices come from doing a simple Google-UK Shopping
search for model numbers.
Hopefully this has been helpful in at least pointing you at worthy possibilities.