The MH Pack Pants get into the same bracket as the Marmot DriClime windshirt-- wonderful design and manufacturing, but they don't work into a multi-day UL pallet. I'd use them both for a day hike or simple overnighter in a heartbeat. In those cases, I don't need to cover such a wide range of conditions.
To make you feel bad, I got the MH pants in a thrift store too-- for $12. I wear XL and they were marked medium. The only thing I don't like is that they are black (no longer offered) and too hot if the sun is out. That is only a problem for 12 weeks or so in Washington :)
The cuffs on the MH pants are great-- ankle zippers and toggled drawstrings too, so I can keep them toggled up and out of the mud (I wear a 30" inseam) and still get the convertible features if it warms up or I need to wade. They aren't light and I wouldn't use them in hot climates.
Shorts are one of the #1 buys in thrift stores and I really have them coming out my ears-- Patagonia baggies, Grammicci, Nike ACG, Columbia, REI, Ex Officio, etc, etc, all for $1.99-$3.99 a pair. I found a pair of Nike ACG shorts that are thin nylon with a liner and the pockets all have zippers. I imagine they were originally marketed as river shorts. I really like the zippered front pockets so things don't get lost. Those shorts like the REI Sahara's with the cargo pockets look real Rambo, but they're kind of like an over done Swiss Army knife--- just too much bling. I have a pair of Nike running shorts with a good liner that I'm waiting for a good hot summer day to use-- like a feather and great ventilation. Bright orange too-- you'll see me comin' down the trail :)
Getting back to layering bottoms: once you include light rain pants it seems to me that you have to build from there. The Marmot Precip pants are thin and light and if it is cold and blowing, I don't get clammy with less breathable bottoms. This is mainly camp and rest stop issues, and/or extreme weather. Anyway, having both wind pants and light rain gear doesn't make a lot of sense. If it's cold and wet, a pair of thin long johns underneath is going to help with perspiration managemtent and keep the cold clammy rain gear off my skin. I wouldn't wear long johns together with briefs more for the friction issue than anything else. Also, the fewer layers under a pack belt, the better. From a weight and simplicity standpoint, light shorts with *good* liners, long johns, and rain pants make a good system. The more I look at it, including briefs are only necessary if you have unlined shorts or pants like the MH or TNF convertibles. Many of the Ex Officio models have liners that won't sandpaper your backside. The Columbia shorts I've seen might as well have 80 grit grinding wheels than those mesh liners they use. It's the fat seams on the edges that get you where it hurts. Nike is better and Patagonia the best. My other pet peeve is waist bands. Elastic waists and drawstrings are terrible, IMHO. The waist is where the MH pack pants really excel, with the soft chamois-like lining, smooth seams and a very thin belt with a smooth thin buckle. Any pack shorts should have thin nylon belts so you can keep them from falling off if you have essentials in your pockets. I say that I'm "all gut and no butt" and there's no place to hang my pants so to speak, not to mention parts of my anatomy that are trying to push my pants down rather than hold them up. Hopefully with some more hiking, that problem will ummmm... fade away :)
I have a pair of Campmor expedition weight polyester long johns that I think will work great under rain pants in nastier conditions and are perfect for sleeping in. They are almost fleece, but not quite, very stretchy and about 8oz a pair. Cheap too.