I wouldn't recommend building your own cabs to somebody new to the business Dan!. No offence to Martin, but if he needs to ask advice on purchasing products, then I don't think he'll be able to design and build a speaker system from scratch, and correctly from individual components!.

Usually the rave industry goes for the pro end of the market such as Turbosound Floodlight speakers driven by QSC Amps, although you would need a lottery win (or grant ) to even consider going for something like that - and for 80 - 100 people then its going to be overkill - just some advice for when your drawing the really large crowds / outdoor events!.

Martin Audio - are a good Pro loudspeaker, and you should be able to get a decent Martin / QSC derived package for around £2800.

EV- Again, Industry standard for this sort of application. Their Eliminator E / SE combinations will fit well within your budget, and if your budget becomes tight then good condition Eliminator 15's can often be found in 2nd hand music stores and cheap!.

RCF - Mackie I don't go for powered speakers myself (personal opinion), and some of their powered products have had issues with overheating especially when driven hard for long periods. However some of the Passive RCF Range is exceptionally well made. The RCF ART 300's (Full Range 12" 300 W RMS) and RCF ART 500 (Full Range 15" 500W RMS) are popular for the full range portion of the Audio.

Amp:- If your budget can stretch to it then go for QSC RMX Series Amps. you can get their RMX-2450 (2x 750W @ 4 Ohms / 2x 1225w @ 2 ohms) for under 600 notes.

Choosing what goes between your mixer and amp rack can also be important, if possible try and use bi-Amping, that means using two amplifers to power your system. For this size of system then I would recommend using one amp for the full range, and one amp powering the bass or "subs". You can use a Active Crossover to do this. Additional (low cost) enhancement which can be added to your system is a Bass (or frequency) enhancer and a compressor limiter. Between them, this will add more kick to ANY system and improve clarity and bass response of the system without using additional overheads, powerwise!. The limiter protects your system from being overdriven, and compression, when use correctly can also add depth to your music output . If you go down the Behringer product route for these, you can get these signal "add ons" for very little damage to your budget!.

Again, Audio is a different Animal to different people, we all have our favourite toys and systems, and idea's of what we ourselves would use (given the opportunity or budget!). All the advice given so far (apart from maybe Dan's D-I-Y bodget and scarper) has been spot on and worth considering, but perhaps you would be better demo'ing a system before you buy since you seem to have your own idea's of how you want it to sound, and the only way of really re-assuring yourself that a system is a correct system for you, is by listening and playing!.

I would go along with what Chris has said, that's why I thought he would be a good 'devils advocote'....but bodget and scarper.....LOL!

I can honestly say, that what I did was only re-case the drivers into a more suitable 'bin' to fully appreciate full power potential for larger gigs, especially in cases of 1000W+ required output. (The RCF drivers had been placed in 'car-boot wedges'!!!)
Actually, I miss quoted myself, and it wasn't rehoused in 120ltr cabs, but 100ltr cabs!!!
So I did bodge that earlier statement!

No offence Chris, but I wasn't under the impression that Martin was 'new' to DJ'ing.
However, I submit, that DIY speaker building from scratch IS more than a potential risk to newbie DJ's - as it puts the life of your amp, and speakers in the balance! Increase the risk - the higher the replacement cost.

Stick with Chris - I'd better scarper!