If I buy the game on Impulse, will I have increased difficulty in terms of installing mods (an ever-present problem on Steam, it seems)?
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No. Impulse is very benign and the only time I ever load it up is when a new patch is released. You may as well purchase over Impulse if that's the best price since you're going to have to install Impulse anyway in order to patch the game, and most of the mods, at least the ones that are well-maintained, probably require having the latest patch.
Is there a guide anywhere to installing mods manually, that is, assuming the mod in question does not come with an installer?
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Numerous posts have probably been written about it. Smart modders will include a Readme.txt file telling you how to install them or even better, an auto-installer file. You can probably find all the information you want in the Modding forum. If you know enough about Windows to be able to find your Sins Application Data folder, you shouldn't have any problems. It's normally as easy as unzipping the mod, opening your Application Data folder, and then adding the mod files to the proper expansion's Mods Folder.
Note that it might be necessary to update your mods each time a patch is released (which really doesn't happen all that often anymore). The modders will update their mods for you. The game has a nice mod system where you choose which mods you want to activate from an in-game menu; it's neat that way. Note that if you decide that you're interested in playing it in online multiplayer you'll probably want to disable all mods since you won't be able to join other people's games if you have the mods enabled. (If you and your friends all have the same mod and have it enabled then you can join each others' games.)
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