Cost of ownership question

I am going to put a lot of constraints on this question, bear with me. How
much does the size of the engine and airframe contribute to cost of
ownership? I am looking at buying a plane as are many of us. I am stuck in
the infinite loop of, well if I spend an extra $5K I can get this...but oh
look, another $5K gets me this and WOW for just another $10K I can get
THIS.....repeat. Somethings are obvious, CS prop more maintenance than
fixed prop. Retrac more maintenance than fixed, etc. But, other than fuel,
is a 180hp much more expensive to maintain than a 160hp or a 115 hp? How
about Continental vs Lycoming vs Franklin vs Ranger radial? I have some
flexibility regarding purchase price. What is more likely to burn me later
on is month to month expenses This is what a need an sensitivity analysis
on.

Thanks.