Honestly, I believe the fear of wart scorching is over rated. I run a 4500 Watt ULD element in my BK and it has plenty of umph to get the kettle to a rolling boil. I wouldn't suggest anything less though unless you know you are never going to move on from 5 gal batches.
I would imagine if there was scorching there would be evidence of carmalization on the element and I haven't seen any. My beer certainly doesn't taste burnt either.
If it is a matter of controlling your boil off rate, an auber PID controller with SSR allows you to set a 0-100% duty cycle output manually. I havent seen this feature in other PID controllers in the same price range. Ought to set you back around $80 bucks. It's what I use and I like it.
you cant get a stainless jam nut and silicone gasket from bargainfittings.com to mount it through the wall of your boil kettle.
It's hard to screw up the wiring. You'll either get 120 across the element or 240
Protect the terminals some how and...
The most important thing is to make sure you bond the ground to the kettle some how. This way if voltage does leak it will pop the breaker and not be sneaky waiting for you to grab it. GFCI breaker is good insurance but I don't think they are as necessary as some people preach. Think about, an electric range top has the element in direct contact with metal pots and are subject to boil overs and what not, they aren't on GFCIs. I'll say it again, GROUND THE KETTLE
Hey Coderange - how long, with your 4500W element, to bring to boil 5 Gallon batches? or do you do 10 Gallon?