First the downsides, why on earth would you want to train on a single speed bike, hills beyond a certain gradient are slow, grinding, gruelling affairs, headwinds provide a similar gruelling experience; marry the two with the hunger knock and you’ll never want to ride a single speeder again.
But there are some positives, single speeders remind you how to spin, they build leg strength, they are great for base conditioning on flatter routes and, they’re cheap to maintain and are generally light and responsive. If you’re climbing a hill where the gear ratio and the gradient are suitably in tune, you’ll find that sweet-spot where climbing with the single speeder is quicker than a road bike with gears.
Here in Scotland, a single speeder spec’d with a free-wheel is a sensible solution to the harsh winter salt and I’ve ridden one for the last three winters. I opted to start from an off-the-peg single speed mount, the aluminium Giant Bowery, and flesh it out with a better finishing kit that is robust enough to handle a bit of mileage.
As with many club-men’s winter bikes, much of this is comprised of old tat. There are some interesting bits though where the primary focus has been on robustness.
SPD pedals are a necessity due to their robustness and are far cheaper at £20 a pair as compared to Look pedals which don’t suffer the salt well.
Transmission wise, the original Bowery 3 x 32 chain and crankset lasted just a-few months before it wore out and was replaced with a heavy duty 1×1/8 chain and a good quality Velo-Solo chain-ring which was paired up with an old set of Campag Centaur cranks.
The Pro Bars and stem are a good cheap option that surprised me with their quality. Especially the stiffness of the stem which resides at the bottom of the Pro range which is great value for money.
The most common wear and tear item beyond tyres and brake pads has been the chain and free-wheel. As mentioned above, get the heaviest duty chain you can and keep it as clean as possible to maximise wear.
I’ve tried a number of free-wheels and see no difference in terms of longevity between an expensive Shimano DX BMX free-wheel which is allegedly ‘sealed’ and a cheaper Dicta or similar product. The Dicta free-wheels are surprisingly tough with very hard wearing teeth and the internals can usually be resurrected even when they sound terrible if they are soaked in heavy grade motor-cycle chain oil.
With regards to wheels, Mavic is an obvious option but I’ve been very impressed with the cheaper Halo range. 
I’ve tried a rear Halo AeroRage with has a sealed hub which is still buttery smooth after two winters of abuse and the rim surface is also hard wearing.
The riding experience is taught and responsive in general but there is noticable flex from the frame under heavy load when out of the saddle. However at this cheap price point you take what you get.
Yes but I want a couple of gears, there’s too many hills round here
When the downsides of single-speed life wear down your mojo, you realise that all you need is maybe two or three gears instead of one.
Whatever way you look at it, adding gears is expensive with options ranging from Sturmey Archer 3 & 5 speed combo’s to Roloff at the top end.
The quality of Sturmey Archer is impressive and the company’s longevity means that spares are readily available making servicing a reassuring option. There is plenty of guidance on-line and the Sturmey Archer blog or good ol Sheldon-Browns are good places to start.
A new custom wheel build with the hub of choice will have to be paired with a gear shifter and Sturmey has the perfect bar-end mount for a roadie:
