Surly bikes rule. Look at the CrossCheck. Probably a little blue collar for this crowd but way functional.

mtbr.com message boards either Singlespeed section or 29er section FAQs will give lots of input on gearing

also roadbikereview.com cyclocross discussions

You can weld on a der. hanger later if you want (with steel) but you will have to strip the paint (it's flamable). Its easier to buy a frame with the hanger on at first and not use it if you don't need it. Look for something like the van dessel country road bob (the new one looks better than the last few years). Or any bike with horizontal dropouts (or nearly so) and a hanger. eddie

You can always get a rear hub with an eccentric axle. That would let you use a normal bike with a rear derailleur.

Or you could get the Kona Sutra touring frame. It appears to have sliding dropouts.
http://www.konaworld.com/Prod/000000199/2K5_Sutra_Frame_CMYK-Listin.jpg

-what's a good gear ratio for flat terrain (44x18 or would a 42 be better)
*44x18 should be doable, the 42 isn't much smaller and would give you some added rpms for good leg speed work. I'm typically in a 64 inch (52x22) gear in the off- and early season, then get bigger as the group rides start running a 48x16 or 48x18 depending on the crowd. Keep it fixed and race some TTs on it. You'll love it. I'll use a 53x16 or 55x17 for TTs.

-if I later decide to switch to a full geared bike, is it possible to somehow add a derallieur hanger...are other mods needed/possible?

*tough to find a NEW bike that will do both, but an older frame might fit the bill with old-school horizontal dropouts. My fixed gear was an old 3Rencho road frame until a couple years ago. I know that the Surly can be run with a SS or geared set up. There are hubs with an eccentric axle and even gear combinations that can be used with a vertical dropout if you want to use a pure road frame.

- anyone familiar with the On-One Il Pompino (don't ask what this means in Ital) - seems like the Uk version of a Surly possibly with a nicer frame
*I've seen a few and they look nice, I second the Van Dessel options also.

Other tips? My priority is comfort, durability and low cost with the clearance to run cross tires w/ fenders
*That is going to be tough to find outside a cross frame with SS options. Are you going to run a rear brake?

-SD

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https://www.kickstarter.com/...bike-for-the-new-era

I would second the surley... well made and has the single speeder in mind with some of their frames...

If I am not mistaken vandessel also has a similair SS/geared appropriate frame but I can't remember the specifics