Hope you all are fine. I have two cisco 3845 with C3845-ADVIPSERVICESK9-M), Version 12.4(3d) at Head office end as Core Router and 200+ cisco 1811 and 30+ Cisco 851 at Branch and Remote location. I am getting bored with creating p2p GRE tunnel with IPSEC at my core Routers. I am afraid of counting the configuration lines. I am thinking about DMVPN. How wise it will be with current Hardware platform. My managemnet is not agree with expenssive hardware as they are in cost cutting process. But I am anxious about sudden collapse of my network. Please suggest me.
as Joseph has noted you should be fine.
You may need to upgrade IOS images on branch routers.
The following link to DMVPN solution reference design can help
notice table 4-3 about performance of routers in a Hub and spoke scenario (it shouldn't be with spoke to spoke tunnels that is in the following paragraph)
With EIGRP you easily get an Hub and Spoke DMVPN without spoke to spoke tunnels: actually some tricks are needed to support dynamic spoke to spoke tunnels ( no ip next-hop-self eigrp ASN in tunnel configuration ).
Only concerns are for the 851
you can check this using
search by feature
digit D and select DMVPN phase 1
in the list of platforms I can see 831, 836, 837, 877 but I don't see 851
Hope to help