As already stated, there are no "must have" effects.
That said, if you like funk, people will look at you funny if you don't have an envelope filter. I personally own and like the 3Leaf Groove Regulator, but there are plenty of other good filters out there. Just find the one that suits your taste in funky Bootsy noises and roll with it.
Also, depending on which filter you get, you may need a compressor or limiter after it to tame volume spikes. For that information, I will refer you to the gold mine of compressor info:
http://www.ovnilab.com
Hope that helps.