Let's not forget that installing SSIS on its own server requires purchasing separate licenses for SQL Server on that server. If you are running SSIS on an ETL server and do not have the licenses required - it could end up costing you a lot more than you realize.

For that reason alone, I normally setup a reporting system/data warehouse and include the ETL processes (SSIS) on that server. This leaves my production system without the requirement for additional components. Since the reporting/data warehouse server is just that - it is much easier to schedule a restart as needed.

For a constrained environment like this - I would definitely recommend the above scenario. However, keep in mind that wherever you install SSIS you need to account for the additional memory it requires. How much you leave for SSIS will be determined by your ETL processes and how they are written and configured.