What yeast you can use
Bakers yeast vs Special yeast (turbo)
Bakers yeast will produce a maximum of around 8% alcohol, whereas the "turbos" can generate up to 20% alcohol. Obviously you'd use different amounts of sugar for either case. Its not that the Turbo makes a higher % from less sugar, its advantage is that it can handle the higher concentrations (first of heaps of sugar, then later, the high alcohol %), and hence you need proportionally less water.
Conclusion: you will end up with more alcohol in your 20L wash when using turbo yeast, because you are able to put more sugar in.
But....only use the Turbo's if you're after a "neutral" alcohol. If you're trying to make a flavoured spirit, (eg corn whisky, brandy, rum, etc) then use a yeast which will help give you the flavour profile that you desire.
While making the ethanol, the yeasts will also generate very small amounts of other organic compounds - including other alcohols, aldehydes, esters, etc. These are known as the "fusel oils" (or the "cogeners"). It is the presence of these that give the alcohol its flavour which we are looking for in corn whisky etc.
All common available bakers yeast are suitable for fermenting the wash. Use at least 150g for a 20L wash.
Benefit of Turbo yeast is that it is pre-packed with all the nutrient needed, so you do not need to worry about what additional nutrients you need to add. That makes life simple, however there are quite some brands available though online stores and local homebrew shops.
Some of the turbo yeast brands available are:
For whiskey (corn or malt) a dry ale yeast is the best yeast type available.
Yeast from commercial beers
There are many quality microbrewed beers on the market that are bottle conditioned, i.e. naturally carbonated and unfiltered, much the same as homebrewed beers are. The yeast layer from a bottle conditioned beer can be harvested and grown just like the yeast from a liquid yeast packet. This is a common practice among homebrewers because it allows for the use of some special yeast strains in homebrew that would not otherwise be available. Keep in mind that some brewers add a different type of yeast prior to bottling.
Harvesting yeast from a bottle conditioned beer is quite simple.
For best results, add the sediment from 2-3 bottles and be sure to use the freshest beer you can find. The starter should behave the same as any other liquid yeast pack starter, though it may take longer to build due to the smaller amount of yeast that you start out with. In fact, you may not notice any activity in the starter for the first couple wort additions until the amount of yeast builds to higher levels. Add more wort as necessary to build the yeast slurry to pitching level.
Best of both worlds: mixing different yeasts