To install the element, drill a 1.375" hole and insert the element. Use the supplied seal and use a 1" straight thread SS nut from Bargain Brew Fitting Home Page
Simple, cheap, effective.
Id go with a 5500W element they are no more expensive than a 4500W and will provide more power for a 10 gallon boil.
In an un-insualted vessel, you will require about 3500W to maintain a vigorous boil with about a 1.5gal/hr boil off rate with a 7 gallon volume or wort. This has been my experience with my electric rig, and we even proved it using mathematical equations in another thread! Pretty sweet.
PID and SSR are totally necessary. You can MAYBE jerry rig something, but you will waste time and money doing it.