I'm happy to try and help out. If you're less than 200 handsets I wouldn't bother speaking to the networks directly as you are probably too 'small fry' for them to deal with...if you want sensible answers from an authorised service provider that deals with 5-100 handset customers then I'm might just be able to help (sorry shameless plug..)
Let's take it from the top.
1) BIS or Internet Mail is where you have a blackberry that is linked to the @mobileemail.vodafone.net. Any emails sent to
(for example) will be pushed automatically to the BB device.
2) Once you have the BIS account setup and tested you can then integrate any third party mailbox with pop3 access (e.g. your own domain email). Then the BIS account polls the 3rd party mailbox on a 15-20 minute basis, collects and email and then pushes it out to your BB device.
3) You can then spoof your reply-too address so that any email sent from the device would appear to come from "email@example.com" or any other mail account you require - i.e so you don't ever give out your mobile email account
4) BES is different - here the devices acts like a 'remote control' to your email account so any emails sent/received via the device are actually sent through your email server. There's 5 other really good reasons to go for a BES solution rather than BIS, but I can go into that later
5) With Blackberry attachments are never actually 'delivered' to the device, instead the BES server (or BIS service) actually opens the email and 'streams it' to the device. Therefore the majority of UK services are sold on a "All you can eat basis" - i.e. you pay a flat monthly rate for 10kb or 10mb or 100mb. You donot need an additional plan for web browsing, the BB plan includes all data usage.
Drop me a pm with your contact details if you'd like a follow-up call or email, or would just want some more questions answered.