You mean 2600s? The 2900s are the switches. I'm not familiar with the ISRs, but I believe the 2800 series are the next step up from the 2600s.
Depending on needs 1800-2800
No. The 1841 is only rated for E1/T1 speeds. It will do more but thats just if its doing basic NAT + routing. Turn on IPS or anything else and it'll drop like a stone. The 2800 ISRs can all cope with multiple E1/T1s.
If you actually want the router to do voice then you have to get a 2800 series. 1800 doesn't support voice. If you want all 80 users then you'll need the 2851 at a minimum handle that many.
How do you plan to load balance? Two static routes? If you are planning on using BGP then the 2800's may not be enough.
I do know of one of our clients that use EIGRP over frame and it works really well. It's not terribly resouce intensive on the router either. That's the route I would go.
Is this a multisite implementation?
If not you are not going to need an IGP over the WAN. You could just use static routes, but if you want granularity and best routing you will need the BGP.
You can use static routes and rely on the line protocol state to determine if the route should be in the routing table, or you can also tie a ip sla configuration to the static route that will ping the other side and remove the route from the table if the pings fail.
Just make a static route to the interface. This will remove the route if the interface goes down.