You can configure the Autonomous Database instance as a target database for Oracle GoldenGate. But You can't set up Oracle Autonomous Database as a source database for Oracle GoldenGate.
Recovery Point objective (RPO) of 24 hours and Recovery Time Objective (RTO) of 1 hour
- To provision new VM and restore the production database from the backup on object storage, will exceed the RTO 1 hour
- You can create the standby DB system in a different availability domain from the primary DB system for availability and disaster recovery purposes. With Data Guard and switchover/failover can meet RTO
1 hour.
- RAC Database is not required in this solution. Standalone will be most suitable and cost effective

MongoDB is a cross-platform document-oriented database program. Classified as a NoSQL database program, MongoDB uses JSON-like documents with schema, so the best to be migrated to Oracle NoSQL Database.
https://blogs.oracle.com/nosql/migrate-mongodb-data-to-oracle-nosql-database Autonomous transaction Processing Serverless (ATP-S) isn't supported yet for EBS database

Based on cost considerations will exclude the Exadata. and there's no need for Data Guard Cost Estimator
https://www.oracle.com/cloud/cost-estimator.html

Explanation
Oracle Cloud Infrastructure offers two distinct storage tiers for you to store your unstructured data. Use the Object Storage Standard tier for data to which you need fast, immediate, and frequent access. Use the Archive Storage service's Archive tier for data that you access infrequently, but which must be preserved for long periods of time. Both storage tiers use the same manageable resources (for example, objects and buckets). The difference is that when you upload a file to Archive Storage, the object is immediately archived. Before you can access an archived object, you must first restore the object to the Standard tier.
you can use Storage Gateway to move files to Oracle Cloud Infrastructure Archive Storage as a cost effective backup solution. You can move individual files and compressed or uncompressed ZIP or TAR archives. Storing secondary copies of data is an ideal use case for Storage Gateway.

Autoscaling lets you automatically adjust the number of Compute instances in an instance pool based on performance metrics such as CPU utilization. This helps you provide consistent performance for your end users during periods of high demand, and helps you reduce your costs during periods of low demand.
you can associate a load balancer with an instance pool. If you do this, when you add an instance to the instance pool, the instance is automatically added to the load balancer's backend set . After the instance reaches a healthy state (the instance is listening on the configured port number), incoming traffic is automatically routed to the new instance.
Instance pools let you provision and create multiple Compute instances based off the same configuration, within the same region.
By default, the instances in a pool are distributed across all fault Domains in a best-effort manner based on capacity. If capacity isn't available in one fault domain, the instances are placed in other fault domains to allow the instance pool to launch successfully.
In a high availability scenario, you can require that the instances in a pool are evenly distributed across each of the fault domains that you specify. When sufficient capacity isn't available in one of the fault domains, the instance pool will not launch or scale successfully, and a work request for the instance pool will return an "out of capacity" error. To fix the capacity error, either wait for capacity to become available, or use the UpdateInstancePool operation to update the placement configuration (the availability domain and fault domain) for the instance pool.
during create the instance pool you can select the location where you want to place the instances" In the Availability Domain list, select the availability domain to launch the instances in.
If you want the instances in the pool to be placed evenly in one or more fault domains, select the Distribute instances evenly across selected fault domains check box. Then, select the fault domains to place the instances in.