Yeah DoF and motion blur are the first things I turn off in any AAA game, since they go ham on that shit. But I wasn't suggesting you be that liberal with it. Selective use in one or two scenes might be warranted though, in such a scenario as a chase or whatever. But I don't know what you have planned for future episodes, so I was just spitballing.
I'm completely unfamiliar with it, only having worked with 3DS Max and Cinema 4D before. But if you absolutely had to go ghetto you could render out some alpha masks simply by applying black or white materials appropriately in order to separate out the elements of a combined render. It could work fairly seamlessly with some careful scene composition.
As to Gimp, it has been a fair few years since I last used it, but given it was always meant to be the open source alternative to Photoshop I dare say it should have equivalents to most of PS's major filters. For example -
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