And, a last one, what do the pre amps do? Give extra power or different (and mixing) tone?
The main benefit of a separate pre-amp is that the processing of the delicate source-signals have a greater degree of isolation from the noise generated in the output section of a power amp (this was much more relevant in the age of turntables, when the source signal was tiny ). The downsides include the need for another set of cables/connections and another set of casework.
On the whole, if you are talking about components costing hundreds of pounds, rather than thousands, the cost/benefit relationship is more likely to favour an integrated amp. Although, having said that, Cyrus (among others) do offer a Naim-esque upgrade path, where you can start with an integrated, then add power amp(s), PSU(s) if required.
In terms of budget split between source/amp/speakers, I think any real truth behind the "Front-End-First" approach was rooted in the analogue age, as the level of mechanical engineering needed to make a decent sounding turntable was and will always be expensive. The economies of scale involved in digital processing now means you can get a half-reasonable sound from CD at a relatively low-cost, so there is a fair bit of sense in putting the majority of your budget towards speakers.
Most important thing (as has been said earlier) is to listen to some alternatives and, if at all possible, listen in your own system rather than just a demo room (some retailers will let you take kit home if you leave a deposit).
Second most important thing is not to fall down the slippery "upgrade" slope if you can help it!