What is your design going to look like? where is all the phone lines? everything at HQ or phone lines at each site?
Are you going to use SRST/CME for backup purposes?
What VGW's are you using at each site?
How stable is your internet connections? at each branch? and at HQ?
I personally have not used IME, but I think it would have a lot to do with the design. Can everything run from branch to HQ to IME, or are the branches all over the world, and for bandwidth purposes need to be closer to the branches?
Budget dollar wise, maybe depending on size of the branch you could use a 5505 vs a 5510 vs 5525 or to save some cash maybe you would be able to use a higher end IOS feature set to get all your VPN and IOS ZBFW? if your talking about 1 or 2 phones at a branch maybe you could use the VPN feature in the phone?
I think you have left a lot of unknowns for anyone to really answer these questions in full.