1900 era FPS mod project - Suggestions for what game/engine to mod Thread

I am beginning a mod about the Boer War, more information can be found here: Moddb.com

I have experimented with modding in the past but very little so I have to learn most of what I need to do from scratch. Ideally the game I will mod will have be as similar as possible to the type of game I want to make, ie have bolt action rifles, realism (like iron sights) and the capabilities to make large WWI style battlefields.

The mod is for non-profit and I would have liked people to be able to play the mod for free, but because I can't make the game from scratch it will have to be a mod, and I can't think of any free games which look suitable so may have to probably use a retail game. I know of assaultcube/sauerbraten and some very very old fps's which are too dated. Does anyone have any other tips or suggestions? I have been looking at CoD and DoD but have not played them so do not know yet if they would be suitable. It is also important that so many people as possible will be able to play it on their computers, so not limited to only really high end computers. Of course the game will need to be (reasonably easily) editable.

I originally planned to use the unreal engine based Red Orchestra: Ostfront which is perfect for my aims, but because I cannot get the editor to launch and the developers do not know how to fix the error I will have to find another game, which sucks.

Thanks for reading.

Edited by: BOEREKRYGER

ro2 or soure engine