Should I or shouldn't I? Car is a 993 targa 80k on the clock and the engine is...filthy.
What are the benefits and what are the risks.
There can be a benefit to having the engine cleaned in your car's case since it is a 993 and is air cooled. If the engine is very dirty (from a heavy build up of dust perhaps helped by a layer of oil from one or more leaks) this can interfere with the cooling of the engine. The engine can emit a very hot oily engine smell too that some find offensive.
Or if the engine is going to be worked on. Doing any work on a dirty engine is always more risky that working on a clean engine.
However, there can be a risk involved from cleaning an engine. First while steam is used, it condenses and of course turns to hot water. This can and will get into electrical connectors where this can lead to corrosion (of the surface kind) that can produce electrical gremlins. (Example? Well, I 'cleaned' my Boxster's engine and even though I ran the engine a long time after to thoroughly dry the engine, a few weeks later the spoiler acted up. It fixed itself after a while and has never acted up again. However, I have never cleaned the engine again either.)
Also, water can be forced into seals/gasket areas which can compromise these. Someone who is real knowledgeable about these engines and has steam cleaned many before with good results is the person you want to use.
Or you can simply use some appropriate engine degreaser, let the engine soak a while, then rinse off the engine with hot water. Little water force is necessary so the odds of forcing water into where it shouldn't be is reduced. But this is the technique I used when I washed my Boxster engine (covered above) and well, still the spoiler acted up.
Generally though you and the car and the engine are better off if it remains dirty.