David A tool is simply a means to an ends. Anyone with experience of an ETL tool could pick up Talend. What you should ideally be looking for is someone with real knowledge of your business domain or of architecting and creating solutions. If you are building a data warehouse then you want someone who knows about dimensional modelling, surrogate keys, factless fact table, role playing dimensions and other such relevant concepts. In addition to these concepts they should have a good knowledge of modeling and designing these systems. You will find a fair few consultancies out there who charge exorbitant fees and send in graduates who have been on a training course but have no real world knowledge. Yes the system they build will work, but it will be inflexible, inextensible and difficult to extend. So when you find your freelancer make sure you give then a good interview to access the extent of their knowledge. Ask them things such as How to implement a type 2 dimension How many clustered keys you can have a table What is a covered key What are the problems of using indexes and how do they effect your ETL routines ? There are positive and negative effects so see if they appreciate this. What is a surrogate Key What level of granularity should your dimensions be at ? etc..
With a poor database model everything will be poor. You can create the best ETL routines in the world, but if your table design is poor then your system will be poor.