Thanks guys for all the input. That was quite fast.
Thanks Rafeeq, just what i was thinking, that GUP is meant only for virus def updates.
Vikram, I must say, those spreadsheets are awesome. I did a rough quick calc, and found that my DB size would be... GULP.... 4.8TB!!! Regarding GUP, yeah, each site is going to be less around 100-150 users.
Jason, as for distribution, it's going to be one tough task. But that I'm planning to go on site by site basis. As for future upgrades, that too would need to be scheduled again on a site by site basis. nothing else can be done about that.
Now about using 5 SEP sites, when I say sites, I mean the SEPM with SQL DB on the same server. So a total of 5 servers, but all at the head office, as replication partners. Now how would the clients be distributed? Need to create a VM environment to check on this, because I dont want to be manually splitting 50000 users on each server. The idea is I just keep adding and they load balance across all the 5 servers. I know there is a load balance option, but that's with ONE DB. Looking at the number of endpoints, one DB won't support that many.