Need answers to basic questions about ODK

Hi ODK Developers,

I am planning to develop a community mobile app which will help very small
Laundry vendors from India(earning $50-100 per month) to continue their
business without loosing their customers to, new app based, laundry
business who are eating their business very fast. If we don't help them now
then they will be perished. (After all Jon Snow helped wildings because
they are human too!!!) I am planning to use ODK to develop this app. Help
me to help them!!

1> How many concurrent request ODK Aggregate can handle? what is the range
of concurrent requests (0-100, 100-1000, 1000-5000) single instance of
aggregate can handle?
2> What are the possible problems with ODK aggregate?
3> How can I connect ODK aggregate with database? Has anyone tried it with
any nosql like gemfire? Has anyone tries it with apache spark?
4> How to send SMS once I receive request to ODK aggregate? Do we have such
integration available?
5> Which cloud service I should opt? Google App Engine or Amazon EC2? Can
one aggregate instance failover to next available instance?
6> I think this Aggregate Vm is not actually free? Why is it asking me for
credit card details?