Hello Michael,
>> Is there a maximum number of VRF's that can be configured on a router and is this figure platform/OS dependant?
yes I haven't a link to a table of these limits but they exist
>> Is there a maximum number of GRE tunnels that can be configureed on a router and is this figure platform/OS dependant?
yes, it depends from the IDB limits a table that contains the limits see
https://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_tech_note09186a0080094322.shtml
>> Can bandwidth limits be set on GRE Tunnels in both directions?
not totally clear to me, you can police or shape traffic on physical interfaces in outbound direction, the traffic classes can represent different GRE tunnels and/or you can take advantage of hierarchical QoS when it is supported
>>
- If IPSec were used instead of GRE, can bandwidth limits be set in both directions?
GRE tunnels are more handy for routing purposes and they can be put inside IPSec, however again as above
IPSec introduces further performance penalties on devices without an hardware based encryption engine
>> IS VRF and GRE functionality available on all routers, from 800's up?
GRE is supported on 800 series routers and above, VRFs may start from something else
http://www.cisco.com/go/fn
search by feature: VRF lite
notice that VRF lite is only a subset of full MPLS L3 VPN that doesn't use MPLS forwarding and control planes
877 and other are listed on supported platforms
Hope to help
Giuseppe
