Both the 7250 and 7520 are of the same generation, the 7250 is the CDMA version and the 7520 is the iDEN equivalent (there's also the 7290 which is GSM). Therefore Verizon will never sell the 7520.
Both models have GPS but their implementation is different. I know you can use the GPS feature with third party apps in the 7520 just like you can do it with other Nextel phones but as far as I know, with the 7250 it's only for e-911.
If you want to use it as a wireless modem for your laptop, I heard this will become available but for now you can't. However don't take my word for it and ask Verizon (I'm not a VZW client). The handset is capable, it's only a matter of wether it's supported by your carrier or not.
If you want to use you BB as a GPS device for your laptop, you'd better buy a real USB GPS adapter dongle (like the one made by Garmin), it'll be faster and more accurate.
For the text messaging part, it's really a joy with the BB as it's way faster to type, send and receive SMS than it is with a typical phone. For IM, you would need a third party software like Verichat but you need a data plan for that. And yes in most cases you can keep your actual voice plan and just add a data add-on, but it depends if you carrier's policies permit it. You could also probably use Verizon's and/or MSN's wap portal though but it won't be as nice.
Lastly, you mentionned MSN calendar... I guess you could access your MSN calendar from MSN Mobile's wap portal but you won't be able to sync it with you BB unless you can sync it first with Outlook but as an MSN Plus subscriber myself, I haven't been able to do it.
Hope this answers you questions... Others correct me if I'm wrong