Actually you don't look for en "engine", but an open source game with an inbuilt editor.
An engine has no graphics, nor AI, and especially not built-in weapons.
Hack, if you won't even program your own weapons, I don't see how you'd be distincted from a simple map-maker.
As a modder who wants to work in a full-3D editor, you'll either learn to compromise to what actually work when tested on your "4 GB RAM machine", or buy 8 GB RAM with a BIG processor AND a mildly-expensive videocard.
