DSL to T1 Upgrade

You're pretty much on-track with that thought process, as you will most likely be using a new subnet for your WAN connection and its supporting services.

Other than that, the main thing you need to be ready for is the kind of connection the service provider is going to leave you with. In most cases, the service provider will terminate the connection with a CSU/DSU, and maybe even a router if it's part of the deal. But sometimes, they will just terminate the connection at the punch block (d-marc), and you will need to provide the CSU/DSU and router yourself.

Make sure you have a couple of x-over cables too, just in case.

Some standard info:
http://www.t1knowhow.com/

Yeah, you'll be alright. As far as the ISA server is concerned, just start at the external facing NIC(s) and work your way back through the ISA settings.

The only other issue that is usually overlooked is how to mask your subnet. You usually get more than one IP with a T1, so you need to decide how you want to use them. Take some time to do the route planning and associated math before hand.