With all the work we've been doing on openwrt lately, this should honestly be fairly simple to walk through. I'm not sure about this week, but perhaps the weekend or maybe next Tuesday?
Adrian is right, though, the router you probably want is the TP-Link WDR4300.
I believe it should be capable of handling the vpn traffic at acceptable speeds. If not, I'm not 100% sure what the other piece of hardware you want is. Ideally it would be something like this:
But that's obviously significantly more expensive than a raspberry pi. I do think the TP-Link routers should be acceptable for your purposes. I even have a handful, so we can test one of mine before you decide to order one (they're like $40-50).
Should be fun!
Max