Welcome, shrinkopath. What's included in the BB data plan is a bit of a grey area. UK Voda's BIS includes 6Mb/month which comfortably covers email and attachments and some browsing (BB compresses messaging data very efficiently). And with 8820/8120 being WiFi, you won't be charged if you email/browse over a router.
Anyway, you probably wouldn't want to spend hours peering at a tiny screen. Most mobile device browsing is just to check a specific piece of info which doesn't require much bandwidth. Caution with Google Maps, though, which is quite data intensive. Maybe try the standard BB data plan for a month and see how much you use?
As to choice of carrier, Orange is obviously easier for you to arrange, but I've heard complaints about their coverage and customer service. The other carriers all have pros and cons, best to shop around, but plans that offer squillions of Mb for a fixed fee probably not relevant here.