Hey Rich.
Id urge you to give it a go yourself.
You can find one of those car wash places that have the warm water pressure
wands.  Or, better yet, use your own pressure washer while hooked up to
your hot water heater.
Pre-spray your engine and bay with a product designed to loosen engine bay
dirt/grime.  Some people even go so far as to use oven cleaner (i would
not) in order to save money.
But I'd suggest a product like gunk engine cleaner.
https://www.youtube.com/watch?v=0fY1e7oW8Ms
Make sure your engine is warmed up (loosens the grime), spray it down, let
it sit, and blast it with a warm/hot spray of water.
After it's dried off you can take the next step and treat the plastic and
rubber bits with something to help them shine.  Please avoid the accessory
belts when spraying any rubber treatment concoction as it could make your
belts slip.
I typically use Meguiars detailer to make the plastic bits look like new
after cleaning.  But armorall or similar products will do the trick.  Spray
it on and give it a good rub to even out the finish.
Cautions:  When pressure spraying your engine bay you'll obviously need to
take care not to saturate anything but the bay itself.  Keep the wand under
control and the rest of your upholstery will thank you.  Don't stall out
when holding the pressure sprayer on things like the engine connector
plugs, the coil pack, or the spark plug boots.  You could introduce
moisture into areas that would make the engine difficult to start/run.
--
Brent Weide
Portland, Oregon