There are several issues here to consider.
First you need to ask yourself:
What kind of information do the customers has the needs to access?
What level of security do I need and are prepaired to invest?
Which level of knowledge does the person have that needs to make these things work.
When you have the answer to these question you are ready to get going.
In my experience you need to identify the resourses (Server/Servers) which the customers nedds to access.
These Servers needs to be masked (NAT/PAT) from the world outside.
When it comes to the CISCO Router/Routers you need Access-lists which blocks unwanted traffic and opens wanted.
You need also an authentification Server which handles the
PPTP requests, this Server needs high speed (network) to
handle the requests from the customers.
The data storage you are refering to maby has a better solution if there is only a question of moving files from/to your storage. It is also the cheapest solution. If it is a SOHO than CISCO PIX is enough.
I would then suggest a FTPS (SSL/TLS) solution with a webinterface och simply FTP client access and create a simple (how-to) guide and emulate the most common errors (Server access error, wrong password error, home-network ISP error) this to clear the most common problems and make sure that customer relationship is running smothly.