Please see sketch attached.
I understand I should not redistribute BGP into an IGP (in my case, OSPF).
That said, what would be the best method to make my internal clients get out to the Internet. Should I run ospf 1 on router 3648 and issue command "default-originate always" in order to propagate routes into OSPF?
Note that for now, I have only connection to one ISP (MPLS), but in the near future I may need to add second router for redundancy and then I may need another ISP connectivity at that point (for that reason I would keep BGP to ISP).
If you are receiving a BGP default-route from your ISP, you can redistribute only this one into OSPF by applying a route-map to the redistribute bgp command.
If you are receiving the full routing table, you can use the default-originate command. If you want to use the always option, you will need a direct connection between the two BGP routers and run an iBGP session so traffic will not be blackholed if your primary router loose its Internet access.
Whatever the solution is, you also need to play with the OSPF metric on both BGP routers for primary/backup route selection on your OSPF routers.