"What should I be looking for in a mountain bike?"

Decent fit, followed by a couple of sets of tyres for the different terrain. (City/Dirt). Double wall rims if you're a hooligan.

Front sus lock-out is much ado about nothing. You won't loose much energy on tarmac with the suspension left to boing around, and you'll have a more comfortable and faster ride on rough areas.

I don't think the Paratrooper will take a rear rack. (No frame threads?) but it will take a seat post rack.

The usual answer to these threads is that the current 26" wheel bikes are the Dahon Jack & Espresso (no sus), and Montague range.

Front suspension for mild trail, city & XC work only needs to be medium travel around 4" (100mm) or less.

"dirt trails with the occasional log, lots of tree roots, steep climbs"

= MTB gearing, 21/24/27 speeds, 26" wheel cross-country hardtail (XC) bike.

Paratrooper, I think. Very versatile, especially if you're a dirt monkey. :-)

I have a Dahon Jack which is a great city/light trail bike, but for rougher stuff front sus is better, and faster, and heavier.

I also have a home-brew MTB with much the same spec gear as the Paratrooper (sealed bottom bracket, Acera, Suntour XCT crankset etc) and it's done three years dirt duty. All of it good. 9 speed drive train on Para means skinny chain = slightly more wear but more gears.

Keep it lubed up and swap the cables out once a year is all it needs. Front mechs fill up with crap, so get replaced annually, so cheaper is better. Head-race bearings also need lube about twice a year or more often if you ride in the rain. It's all easy to do.

Don't forget front suspension forks need servicing.

Here's the NYCe bikes vid, with great helicopter shots: