For the amount of mileage you are talking about (20-30 a month is minimal), if you can find the proper gear ratio, a half-link will be fine. My wife's townie has been set up like that for months and months and sees more mileage than that and the chain is still tighter than I'd like. You'll need to use a calculator to figure what gears will work with your bike. You can scroll to the bottom of this
SS conversion article I wrote for a link to the calculator (plus info to a bunch of other stuff you asked). Yes, chains "stretch", I do it all the time, but it also is dependent on how you ride and how much mileage you are putting in.
If you need a tensioner, the poster who suggested a derailluer is spot on. You probably have one on the bike already. Spacers from an old cassette or PVC pipe cut to size and an non-ramped (SS specific) cog for the rear will set you on your way.
Conversion is easy but you may need a few specialty tools that you don't already have such as a chainwhip and a cassette lockring tool.