Should I get a power meter? I want a cheap one that does its job; I'm not interested in a $1800 übermeter installed in the rear hub or whatnot. Rear hub Shimano compatible I guess, but then you know... can I replace the freewheel? Is it a good freewheel? etc.

There's those handlebar ones, but I hear they're not great.

I just sometimes feel that when my hybrid hits the back trails, I'm putting a hell of a lot more force on the pedals and keeping my speed constant... 14mph on level mud/crushed rock/dirt at around 90 cadence (I back off slightly, I still need a cadence meter)... whereas on the road I seem to be all over.

It's come to my attention recently that I have no clue what I'm doing going up and down hills, sometimes pushing too hard, pedaling too fast, draining myself to near-death and heart failure climbing hills; and then pedaling hard downhill while recovering. Even on somewhat level ground I'm not sure I keep a good pace, and I feel like I'm doing less work and getting more tired than when I'm burning calories off road yet handling it just fine.

It seems to me that if I'm putting out less power, I should not get more tired; and if I put out more power, I should not get less tired. I cannot imagine any other direct measure that will tell me exactly what load I'm putting on my body. Speed doesn't do it. Cadence doesn't do it. Body temp, heart rate, whatever, those won't do it. The mechanism is constant: my body is producing X amount of power through a fixed mechanical means, end of story. That's the most meaningful measurement I can think of.

Do I need one? Should I get one? What should I get? I don't want to spend over $1000 on something like this. Something cheap and good. Used is good if the unit is durable.