I would borrow a good pressure washer and do it at home. I have not used oven cleaner, but I've used DEP industrial degreaser from Home Depot. The truck I'm working on right now is an ex-farm truck which had its fair share of oil leaks mixed in with heavy dirt...
I also used a pair of wire wheels and good old fashioned elbow grease. It took me 4 days to finally clean the engine and some of the frame, but the complete compartment is not yet done...
I have used Rust Encapsulator from Eastwood and I will use that on the rear frame of my truck. The front is pretty clean so I just sprayed it with Rustoleum.