I converted a 1984 Trek road bike to a fixed gear a few years back. It's not too hard. You just need to need to learn what you're doing.
Personally I wouldn't convert a bike to fixed or single speed unless the drivetrain on it has failed.
What you gain:
- A bit of drivetrain efficiency (Maybe 5-10%. It helps ONLY if you're able to pedal at a reasonable rate - around 90rpm)
- Fewer parts to wear out
- A different pedaling feel (if you used fixed. no different for single speed). It's more direct since there's not the play from a ratchet in the freehub, plus the pedals kept moving by the wheel when you are riding. The 2nd doesn't really help anything, it just feels different. Some people say it helps you get better at pedaling.
- Better for riding in rainy/wet weather (This applies in the winter/spring, if you live in an area where they put gravel on the roads in the winter. That gravel feels really nasty in the drivetrain of a geared bike.
What you lose:
- Being able to pedal at a reasonable rate at a variety of speeds
- Likely knee issues if you spend a lot of time pedaling hard while going slow (if you go up a lot of hills )
- Being able to turn tightly at speed (only if you use fixed, no different for single speed)
- Having to also carry a wrench in addition to your normal flat tire fixing supplies
Basically the differences between a single speed and fixed gear are listed above. With a fixed gear, the pedaling is different with fixed (and maybe very slightly more efficient), you can't stop pedaling, and you can't turn tightly at speed
1 - Whatever you do, keep a front brake on the bike. If you used a fixed gear, you can get used to braking the rear with your legs.
2 - Be careful when cleaning/lubing the chain. It's easy to run your finger into the ring/chain if you don't pay attention. A lot of people have hurt themselves this way. This isn't an issue with a derailleur drivetrain because the chain tension is created by a spring in the derailleur. For a fixed/SS, the chain tension is locked and if your finger goes in there, it will hurt very bad.