I'm all for saving a buck, but this falls under the "Penny wise, Pound foolish" category. I wouldn't buy this guy's "plans". If there were a set of public plans that had been well vetted, then maybe it would be OK, but the only public attempts I've seen on here have been pretty poorly thought out from the safety and compatibility POV. I wouldn't touch them.
If you REALLY insist on doing it yourself, then go the OpenEVSE
route, which IS well vetted. It won't be cheaper than the upgrade, but you have the advantage of being able to build a more capable unit than the upgrade for much less than an equivalent commercial unit.