Hi X-Sauto,
This is a pretty big question- and depends on many things.
I guess a starter is - look at the previous posts, you'll see lots of good advice and comments. Also look at the "how was it for you" posts - people will be posting their experiences.
If you're using MP3/WAV format, things could be complicated by a new Licence called "Pro-Dub". There is a specific licensing thread which covers this in much details.
Lighting is a interesting topic - us "DJ's" love getting new lights (well I do!!). However, most clients don't actually notice if they are DMXed, providing they turn on and off with the music.
My personal advice - get DMX lights in pairs that you can run in Master/Slave. They'll look neat, and both will be synchronised. When you feel you've done a few gigs, then look into a DMX controller.
Sound wise, there are many options (active/passive), and also a choice of sizes and models. Check out the equipment review posts.
Your choice of amp+speakers can be set by the type of gigs you plan to play. If you're looking to do 18ths/21st/16th etc, then they'll probably want it loud and bassy (so budget for a sub/s) Bear in mind those gigs may not pay as well as more adult parties!
30th/40th+ parties may be more restrained, with ~100 people, some of who may want to chat - so a good 300W RMS Pair may be fine.
From a paperwork standpoint, you'll need to be organised - lack of contracts/booking forms can leave you open to abuse (ie, "we asked for 1am finish, not 12am!!!") Some people just turn up and play, others will need full payment and a signed contract before leaving home.
You'll also need to register with the Inland Revenue, either as a partnership, or a sole trader. Consider what will happen if one of you does not want to DJ anymore, or you fall out? Would you be able to put your differences on hold whlst you complete the booked jobs?
You'll also have to complete your tax returns and could be liable to pay income tax and ni on your dj earnings..
However, if you note down all expenses, your costs will be very high, and declared profit very low for a year or so.
When you're ready to start gigs, you should also have Public Liability Insurance. You'll note the "DJ Association" has a low-price deal for this, and its worth checking out the site;
http://www.djassociates.org/Some venues will require this to play, but its really just insurance - if a speaker falls on someones legs, can you afford to pay their loss of earnings+what ever else they come up with?
To finish off - from a quick tally, you've got at very *least* £1,500 to spend. That would be budget gear. You should also get a stand for your speakers, and your set-up.
With this kind on money, you should read the advice given, check out previous posts and I'd recommend visiting a local DJ store who may be able to cut some deals... Simple things like buying a pair of DMX lights and asking for a free DMX cable to link them! They'll be deals on speakers+amps+speaker stands, and its probably a good idea to get the parts in stages (separate visits).
Welcome to the forum!
Jason