I was looking at the treo line awhile back myself, here's my reason for choosing BB: Push Email! That is what did it for me. Not to mention a few places I go, mainly for government b2b type things, I can not have the camera with me, so that means leaving the Treo in the car.
T-mobile, to be honest I wouldn't switch to, in CA they'd be great, but say you travel to some weird location in nevada, service will not be of use. Don't get me wrong, T-Mobile will provide the best monthly price, but not the best coverage. If you work IT, you're probably going to want to stay connected to your email and get updates as they happen, my recommendation avoid T-Mobile (for now). T-Mobile however, will provide you nice, almost blanket coverage in europe for your traveling needs, and the 7100 will cause you to avoid renting a phone, rather use the 7100 with a european pre pay sim card, which you wouldn't even need if your company is willing for some out of country roaming charges.
for the spam issue, set up filters on the BES side in your webclient that your company should provide you with, not only will it allow you to check email if you're in a location with no service, but it will also allow you to set up filtering, so it's not sent to the BB, but rather lefto n the server for you to mass delete later. Filtering could also send it straight to trash for you in most cases.
The 7780 isn't a bad device, I use the 7280 through AWS and LOVE it, with the Easy Speak BT200 from Jabra, the Bluetooth problem is solved for me, I can use bluetooth headset without problem. While it does lack bluetooth data transfer, that's fine, don't really use it on my v600 either.
Over-all my recommendation:
Avoid t-mobile unless you're in a covered area 90% of the time, while they're excelent overseas, they're spotty here in the states, especially the midwest.
Avoid the Treo, again not only will you be unable to enter some secure locations with it, you will get hassled about it at some airports (been there, done that, not fun). It also doesn't provide push email, it must be set to pull ever so often, or manually selected to "get email".
Over-all to be honest I think for your application needs, the 7780 with the Easy Speak BT200 from Jabra will suit your needs just fine. As of now it looks as if the bluetooth in the new models coming will only provide headset profiles anyway.
By the way, welcome to the forum!