1) Gearing: 32x20. Done.
2) Frame material: Steel. Done.
3) Suspension: No. Done.
4) How to tension the chain: well, that's a longer story.
Without a rear derailer, you need some way to keep the chain taunt so it doesn't fly off. Most single speed frames have some sort of gizmo to adjust the distance between the crank and the rear hub, so this length can be adjusted to compensate for different gear combinations while keeping a tight chain. Single speed message boards are filled with discussion about the best way to do this, and here's my take, based on the many, many bikes I've had. These are listed roughly in the order I like them, my favorites to types I will never use:
1) Paragon Swinging Dropouts: As featured on my latest Twin Six:
These check all my boxes: They're easy to adjust, with just two big bolts to loosen. They stay adjusted and don't squeak. The integration into the frame is clean and smooth. The adjustment range is adequate, about 20mm. Wheel removal is quick and easy. As a final bonus, they're made in the US, and replacement dropouts only cost about $15.
Bad points: finding a frame that uses them
2) Paragon and similar sliding dropouts: As featured on my Soma Juice:
Good: two big bolts to adjust like the swingers above, they hold well and don't creak. They have a bit more range than the swingers, maybe 30mm or so. Lots of bikes have similar dropouts, so they're easy to find.
Bad: they look a little clunky, and also require adjusting the horizontal set screw to hold the position.
3) Niner BioCentric: As featured on my Niner SIR9:
This has one bolt that pulls an eccentric insert tight against the sides of the frame's bottom bracket shell.
Good: super easy to adjust, just one bolt (I think the BioCentric II has two bolts, though), holds tight and quiet. It's fun to play with your bottom bracket height, looks very clean, easy wheel removal.
Bad: some people complain about squeaks, though mine never did. Some also don't like the changing bottom bracket height aspect. All EBBs will have a limited adjustment range.
The above systems are my three favorites. I would happily take a frame that used any of these systems. The next group is acceptable for me:
4) Salsa Alternator dropouts: As featured on my Mukluk:
These work basically like the Paragon swingers above, but are more difficult to adjust, and aren't nearly as well integrated into the frame's lines.
5) Split Shell EBB: As featured on my OS Blackbuck and Gunnar Ruffian:
The bottom bracket shell is split, and two bolts hold the eccentric bottom bracket insert in place. Kind of like an old school stem clamp.
Good: very secure fit, since the insert is supported evenly around almost its entire circumference. Clean appearance, etc, as mentioned above for the Niner.
Bad: fiddly to adjust--you have to work at two bolts on the bottom of the bike, where you hand hits the chainring while tightening the bolt. Apparently, the frame machining required for this type is quite precise (=expensive), so you don't see it very often.
6) Set Screw EBB: As featured on my Singular Gryphon:
Two grub screws secure the eccentric insert in place
Good: easier to adjust than the split shell EBB
Bad: many report of owners who tightened the screws too aggressively and either scored their insert too deeply, or deformed their whole bottom bracket shell out of round. Oops. I never had any issues, but I didn't have my Gryphon too long.
7) Chain Tensioner: As featured on any normal frame converted to single speed:
There are many types of these widgets. Some are spring loaded, some are fixed, some are expensive, some are cheap (above) or even homemade (below):
(I won the inaugural Ohio Single Speed State Championship (B division) with the above tensioner)
They all take the place of the rear derailer, taking up slack in the chain.
Good: works with just about and frame, cheap, could even use for a full suspension frame (but see rule #3 at the start above).
Bad: single speed purists will sneer at you, not the "clean" look everyone likes, can't run as a fixed gear. More drivetrain noise. Your chain may derail if your setup isn't good.
Tensioners get a bad rap, I think mostly because they're often a person's beginner step into the single speed world. But I would happily run a tensioner before any of the below setups:
8) Eccentric hub: I had a wheel built around this that I used on various frames:
The hub centerline is offset from the mounting bolts, so you can adjust your tension that way.
Good: converts a regular frame into a single speed without any of the drawbacks of the tensioner. Made in the US, beautifully finished.
Bad: fiddly to adjust, expensive, can slip, just use a dang tensioner already.
Fun fact: I "un-eccentric'ed" my eccentric hub and used it for many years on my fixed gear Cross Check.
9) Bushnell EBB: As featured on my Gary Fisher Rig:
Good: clean lines, etc. Smart people like Jeff Jones build bikes with them, so someone must like them.
Bad: Requires frequent disassembly and regreasing, but it will still slip and squeak.
10) Bad Sliding Dropouts: As featured on my Voodoo Dambala:
Two smallish M5 bolts hold the droputs to the frame. In theory.
Good: they sometimes hold their position. Voodoo has a cool logo and head badge.
Bad: they slip. They creak. You'll spend your time trying various message board fixes, such as scuffing the frame paint, split washers, or bigger bolts. None of it will make much difference. Oh, and they're expensive to replace when you bend the soft aluminum. That's nice.
and my least favorite method of all time:
11) Track ends: as featured on many frustrating frames, such as the Surly Karate Monkey or this On One Inbred:
A horizontal track end allows the wheel to move back and forth to adjust the chain tension.
Good: simple, light, cheap, no moving parts. You'll get lots of rest breaks when you stop to adjust your wheel AGAIN.
Bad: your wheel will slip. Repeatedly, even with a good Shimano QR. Maybe a bolt-on hub works better, but do you really want to carry an open ended 15mm wrench with you everywhere? You can add chain tugs to counter the slipping, maybe even one on each side if you're really unlucky, but that defeats the "simple, light, cheap, no moving parts" aspect of this. God forbid if you combine this with a derailer.
The worst thing about this type of dropout is that many otherwise nice frames apply them, so I'm constantly tempted to try it again, only to get burned. Nevermore!