First, get rid of that kn, lets a lot of dust and water through when it is put under vacuum ( turbocharged diesel).
Second, a bit of smoke this time of year is perfectly normal and should not alarm you. Also, 4 gp's not working good will cause excess smoke and longer start ups.
However, If the seller replaced only 4 glow plugs, then I would be wary of what other maintenance he neglected. I would replace all 8 Glow plugs with motorcraft zd 11's, and while you are in there check the valve cover gaskets for burnt, crispy wiring, If any are suspect replace.
Be wary of Magical Injector cleaners that will net you 100 mpg and another 1,000 foot pounds of torque, many have had success with rev x, etc, I do not have an opinion.
As far as the acc. pedal, you could have a throttle postion sensor going out, others with more experience can chime in.
NO, cruise will not help with Idle. If you want a high idle, get a ts six position chip, and have a custom tune burnt for both the idle and performance. Before you get a chip though you NEED gauges. boost, egt, and trans are a must.
You can also do a diy idle control, as well as a factory option.
check all of the fuses, brake lights ( especially the third brake light) and turn signals as they all can screw the cruise up.