sipXecs with Branch SRST

[BAN]

Good Day,

I would like to know if there is anyone out there that can help me with an Enterprise Design?
I will have 2x sipXecs Servers at HQ (Clustered) for redundancy purposes.
I will have 64 Branches, consisting of 10 max phones and the branches must have SRST should the WAN line go down. I'm not 2 worried about scalability at the branches, so 1/2 PSTN lines are more then enough.
Obviously the solution should have costs to the minimum.

BRANCH:
The idea I have is to have 17XX Cisco Routers at the remote branches that will handle the WAN connections and SRSTv.2 where the IP Phones (SNOM or Grandstream) will be registered with the sipXecs servers and then with the Cisco 17XX Gateway whoudl the WAN go down.
I'm not 100% sure if this will be the most cost effective solution, but thinking that the cusotmer will need a router for the WAN part spending a bit more can give them an all in one solution at the branches?

HeadOffice:
Have 2 sipXecs Servers in a clustered fashion.
I was thinking of instaling a PC with Asterisk and then E1 PRI card in, but again, this branch needs a router so I was thinking of installing a 28XX/36XX Router with Serials, E1 Pri Wic, GSM Wic/s for GSM calls.
The HQ site has 5mb Internet and 5mb MPLS Link, should you want to know why this model router.
This router should have enough power to accomodate this and should still have plenty to spare.
They have already invested in Cisco Switches, so QOS can be setup from the Switch level and thus should eliviate unnessesary processing power from the routers at the branches and having Cisco on the WAN will just make the configurations and compatibilities much easier.

I want to provide to my customer a feature rich and stable VOIP Solution.

Maybe you know of a much better solution and of products that I am not aware of that can help me achieve the same goals.

Thank you Very Very Much in Advance

Rgds
BAN