Should we use Remoteapp or Sessions
Use what you want to use based on your requirements. Without any details we can't give you a recommendation.
Understanding and deploying TS gateway
It is an application-level proxy. Perhaps you need to be more specific about what you need to know in a seperate question?
Would it be ok to have this new server running terminal services as our BDC?
A terminal server should NOT be on a domain controller. You could screw up the domain, there is the potential of a user being able to perform attacks against your DC. Instead of worrying about remote attacks, you have to worry about privilege escalation attacks. For terminal server to operate security settings are lowered.
FYI: there are no BDCs in a 2000+ domain. Every DC is pretty much equal.
Can we use remote apps on xp machines, heard only for Windows 7, need to verify.
To access RemoteApp programs that are deployed as .rdp files or as Windows Installer packages, the client computer must be running Remote Desktop Connection (RDC) 6.0 or RDC 6.1...
The RDC 6.0 software is available for use on Windows XP with SP2, Windows Server 2003 with SP1, and Windows Server 2003 with SP2.
How does SAV work with individual sessions (viewing them as virtual desktops)
How should we setup our profiles for osts?