k5 is an excellent bass. Some may prefer pther pups or preamps. But is a fine bass.
For effects the most useful to me as newbie was: Comp, sustainer, overdrive, phaser, bass wah. i later became constant overdrive user as main effect. Multi effects if you can find one that has suitable version of your fave effects. Is best way to go imo. These also allow for excellent signal to mixers and recorders. Just create your ready for recording tone with the processor. Seperates are lot more expensive and are only better when you cant find a processor with suitable version of the desired effect. Like keyboard synthesizers, the presets on multi effect units are mostly useless. Some being suitable for tweaking and growing your own versions from. The rest just for overwriting with something better for you. Boosting the output level above the preset is a basic key to improving the sound on all multi effect pedals Ive tried with no exceptions. I prefer upper end zooms, line6 makes some nice ones to. Havent been impressed by Boss, or Digitech multis even though they have some very nice seperate pedals.
Most useful effect for bass? To me its overdrive with a very good multi eq after the overdrive. I allways run comp, overdrive, ring mod, eq, and a little bit of room reverb for room ambience (My out the processor is the ready for recording and sending to FOH/PA one).