Change engine harness
 

Hi,
I'm driving 1997 Boxster 2.5 manual with 100k miles. Lately it started to misfire randomly after warmed up and mechanic told me that it needs new engine wire harness cause we already changed ignition coils and spark plugs. MAF sensor, O2 sensor are also changed 3k miles ago. So I also believe that it needs new harness. My mechanic told me that he would probably need to drop the engine to change that but I heard from another mechanic that it can be changed without dropping engine. So, my first question is which mechanic has right assumption.
My second question is regarding which harness to put in there. There are used harnesses on eBay for 100$ and there is new one from factory for 1200-1700$.
So, I was thinking if changing harness does not require to drop engine, I would try with used one first just to check is it really it what is causing misfiring and then after I would order new one.
If engine drop is absolutely necessary, then I would have to order new one right away.

No check engine light? It should and you should be checking the code for diagnosis. Bad harness means no connection in some wires? Should trigger a code or more. And you should be able to repair or replace bad wires, instead of replacing a whole harness.