I'm researching the implementation of a paid membership system and considering customising either the SS eCommerce module, SilverCart or rolling something customised.

Membership will be paid on a yearly basis with the option of an enhanched listing. I couldn't find any default support for the above, but does SilverCart support the following:
+ Recurring payments?
+ Purchasing of digital products?

Hello Marijn,
SilverCart does not have a digital sales or recurring payments feature.
I realize that Silvercart.org is missing a feature description site. Your request just pointed that out, thanks.
The feature description is curretly harboured in the docu:
http://doc.silvercart.org/doku.php/silvercart:20_userbase:20_features

Hello Marijn,

so far SilverCart does not support recurring payments or purchasing digital products.

As for recurring payments, there is more information needed about the implementation. You can e.g. handle this via PayPal externally. See https://www.paypal.com/pdn-recurring for more information. It also depends heavily on the payment system that shall be used for the recurring payment. Since the implementation is depending on the specific needs of the client, we have not published such a module yet.

Digital downloads was on our roadmap but we scrapped it because of lacking demand.

Not sure if you can benefit that much from SilverCart if you are developing a membership based system, since it is a little bit off of the business case we have in mind for our eCommerce module ;-)

Cheers
Ramon