I am planning to convert my 1980 soviet ten speed racer into single speed.
the main reason is that the gearing, 53/43 and 13-14-15-16-17, when set at 43/17 feels heavy, especially since it is not always flat terrain I am riding.
For quite a while now, I have never switched gears. the chain stays at 43/17 all the time, so I think I have been practising single speed.
At the moment, I am thinking of the simplest and cheapest way, by cuttung, and shortening the chain, and eliminating the derailleur.
Soma Rush, Fuji Cross, Raleigh Technium Mtn Bike, Ross 10 speed with Shimano 600.
that is the simplest way. It will do the job for no money. The drawbacks would be less than perfectly straight chainline, increased weight of cogs and chainrings, and the aesthetics of having extra cogs and chainrings. the chainline is probably the most significant factor.
to do it "right" you would need to redish the rear wheel, and replace the existing freewheel with a 17 tooth freewheel. remove the large chainring, and replace the stack bolts holding on the chain rings with short ones. you might also need a different bottom bracket to get the chainine perfectly straight.
None of this is terribly expensive or complicated to do but the cheap simple solution of removing the derailures, and shortening the chain may prove to be sufficient.