I'm not a Linux guru, and never heard of strongswan until you mentioned it. From a quick Google, it's IPsec for remote access.
The issue with IPsec is, unless you have a commercial solution that comes with a client (Cisco, probably others), there are issues getting client software on Windows machines (and I assume that's the majority of what you'll need to support). There is the Shrew Soft client, and I know the author hangs out on our mailing list and people do use it with pfsense. http://www.shrew.net/
OpenVPN is more convenient, IMO, because you can use a single client across every platform you need to support (Windows, OS X, BSD, Linux). With IPsec, you would have a different client from a different source for every platform (again, unless you had a commercial solution).
If I was going with a large scale open source deployment, I would go with OpenVPN in most environments.
For around 100 simultaneous connections, I would go with a Pentium 4 or better box. That should leave you plenty of power to spare.