Don't go with BT for your internet, or at least get other quotes - they are much more expensive than other ISPs (something like 50% as much again) because they have a 100% SLA, which is overkill for a school. We're with Vaioni and they've been excellent for us over the last two years, no complaints at all and great value. Smoothwall for filtering is magical.
Why Linux? If you're going with EES for Office then you're licensed for Windows anyway, may as well stick with what you (and everyone else) know.
Backup software? We're using
DPM at the moment but it is a bit of a PITA, but then again so is any backup solution.
DPM is at least cheap for schools.
Hardware, I've used Novatech & Stone in the past and got great value for money, SwitchShop are who I go to for network kit, KPMS are who I use for audio-visual (they're Yorks based so may not be relevant to you dependin' on where you're at). Bits and pieces get bought from eBuyer, Insight, BT Business, Misco, CPLTD, Millgate... whoever's got it in the cheapest. Ask if your department can have a procurement card, makes it much easier to get bits if you can buy with a credit card online.
Software, buying Adobe Creative Suite is well worth considering if your departments have a need of it, it was about £5k for CS5 for me but that was a site licence for Photoshop, Dreamweaver, Fireworks, InDesign... so much of it gets used so often that it's more than worth the cost.
Moodle for a VLE, with a partner who are alright, I guess.
Consider writing an AUP, and even an SLA if you're feeling like a glutton for punishment.
If I think of anything else I'll come back, I think that's covered most the bases I've been through in the last three years of upgrading from 2003/XP to 2008R2/Win7 (and upgrading the network switches, the server infrastructure, the internet, implementing the VLE...)