GT Outpost is quite OK to use as an everday utility bike.
Turning it into a 1x8 speed gearing is a good idea BUT you may need to keep the front mech as a chainkeeper to prevent it falling off the chainring.
You can replace the rear deraileur and shifter, they are all standard items from any bikeshop. Make sure they are broken before getting new ones. You may need a new rear cassette and chain as well if they are worn. The rear derailleur IS the chain tensioner.

You can fit some fat slick tyres such as Schwalbe Big Apple or some extra puncture protected such as Schwalbe Marathon Plus. I use Big Apples and they are very efficient fro cruising around town.

The saddle is a personal item, cant recomend any particular model.
Riser bars are good. I use On-On Mary, it may be available in the US but there are similar ones around.