Starting from another overhaul mod is not recommended, especially if the mod changes skill tree and/or experience curve - this will result in broken skills and leveling. Starting from existing vanilla save is possible, but some of your equipment might get weird stats after installing the mod (newly found/crafted equipment will have proper stats). Starting a new game is highly recommended both to avoid problems and to get used to mod changes. The mod supports all DLCs, but doesn't require them to function properly. Large portion of the mod is dedicated to resolving these issues: fixing bugged skills and items, providing better descriptions, tweaking useless and overpowered skills and items to be more balanced, changing loot so dimeritium plate can no longer be found inside poor peasant's shack, etc. Vanilla game has lots and lots of gameplay issues: skills not working as intended or not working at all, descriptions being wrong, decoctions not doing what they're supposed to do, quest levels and experience being weirdly distributed, etc. The mod aims to create more challenging and balanced experience, while not deviating too far from vanilla system and principles, i.e.

Ghost Mode is an overhaul mod which changes almost every aspect of gameplay: from combat and damage to loot and economy.

Witcher 3 potion mod