Internet connection failover and mx records

Here's the scenario:

Client has 2 sites that are far apart, each site having 1 Internet connection with intentionally different ISPs.

The 2 sites will  be linked together with a private "site to site" 2mbit link presented as ethernet at both ends.

The first site has an exchange server accepting incoming mail via smtp. At the moment, the 2nd site accesses their email from Outlook using ssl over rpc. Not an issue in itself but the country the client operates from sufferes from slow, unreliable (and costly!) internet connections that are 256Kbit/s at best.

The intention would be to have the mx records for the exchange server set up as follows:

priority 10 on the static, public IP 1st internet connection (existing)
priority 10 or 20 for the static, public IP on the 2nd site's internet connection*
priority 30 for the ISP's mail server on the 1st site's internet connection**

*The smtp traffic for this will obviously have to go through the "site to site" link to reach the exchange server.
**This is incase the exchange server itself is down.

I am intending to provide provide fault tolerance for both site's internet and email access by installing a router in each site that has load balancing/failover wan links such as the zyxel zywall 35. That way if site 1's internet connection goes down, site 2's is available for all.

As an additional benefit, if the dedicated link goes down, site 2's Outlook users could hopefully still access their exchange mailbox. However I'm not sure on how dns would behave as the internal and external fqdn for the rpc proxy resolve to different ip addresses (192.168.x.x and 195.x.x.x respectively). Should dns caching on the WinXP client PCs be disabled?

I basically need the following answered:

1. Are my intended changes to the mx records sensible?
2. Is the Zywall 35 a good choice for failover applications such as my situation?
3. Is it feasible for site 2's outlook users to be able to access the exchange server in the event of the "site to site" link being down?

If anyone can answer 1 or more questions I will gladly split points- I'd offer more points if I was allowed!

Yours hopefully,

Jon.