There are several dependencies between the standards in the ISO2700X series also called the "ISMS family of standards" that are not clear from the beginning - so your question is absolutely justified.
Fortunately there is a Figure for that:
Source: ISO/IEC 27000:2016
What every single standard does can be somewhat inferred from their names. So here's a
- ISO/IEC 27000, Information security management systems — Overview and vocabulary
- ISO/IEC 27001, Information security management systems — Requirements
- ISO/IEC 27006, Requirements for bodies providing audit and certification of information security management systems
- ISO/IEC 27009, Sector-specific application of ISO/IEC 27001 — Requirements
- ISO/IEC 27002, Code of practice for information security controls
- ISO/IEC 27003, Information security management system implementation guidance
ISO/IEC 27004, Information security management — Measurement
ISO/IEC 27005, Information security risk management
- ISO/IEC 27007, Guidelines for information security management systems auditing
ISO/IEC TR 27008, Guidelines for auditors on information security controls
ISO/IEC 27013, Guidance on the integrated implementation of ISO/IEC 27001 and ISO/IEC 20000‑1
- ISO/IEC 27014, Governance of information security
- ISO/IEC TR 27016, Information security management — Organizational economics
Sector-specific guideline standards:
- ISO/IEC 27010, Information security management for inter-sector and inter-organizational communications
- ISO/IEC 27011, Information security management guidelines for telecommunications organizations based on ISO/IEC 27002
ISO/IEC TR 27015, Information security management guidelines for financial services
ISO/IEC 27017, Code of practice for information security controls based on ISO/IEC 27002 for cloud services
- ISO/IEC 27018, Code of practice for protection of personally identifiable information (PII) in public clouds acting as PII processors
- ISO/IEC 27019, Information security management guidelines based on ISO/IEC 27002 for process control systems specific to the energy utility industry
You don't won't need every single one of these. It is best to start with the ISO27000 to get a good overlook. All the dependencies within the ISMS family of standards are explained here. Luckily this standard is available for free on the ISO website, although a little hidden1. After you have understood what you want you should know what standards to buy.
But, the important part here is this: the cost for buying the standards is probably insignificant (100 CHF+ per standard) in comparison to all the cost you will have when implementing your ISMS and getting ready to be audited. This is a very long and somewhat expensive process. Worrying that the 200 CHF you will maybe pay the ISO is too expensive, is the wrong mindset for this.
1 You can visit this site: https://standards.iso.org/ittf/PubliclyAvailableStandards/index.html or look for "Publicly Available Standards".