SQL Server Maintenance Plan - Multiple vs Single Plan / Cleanup History
First Answer
It depends, i.e. number of databases, database size (maybe all database might not need daily backup and log backup). However, for flexible and reliable approach I would refer this solution.
Second Answer
The retention period must be decided based on the requirement of business and nature of the data, most cases 30-60 days retention period followed, but again it depends on your environment and business, to get an idea with the backup storage requirement quickly, take the following query result into excel, hope you can easily analyze to get average backup size per day, per month etc..
select    backup_finish_date
        , FORMAT (backup_finish_date, 'yyyyMM') as backup_month
        , database_name
        , cast( (backup_size / 1048576) as numeric (12,2)) backup_size_MB
        , cast( (compressed_backup_size / 1048576) as numeric (12,2)) compressed_backup_size_MB
from msdb.dbo.backupset

Note: BACKUP Compression not possible in SQL Express edition