Unknown year gt aggressor pro I got free from a customer. It actually rides well for around the city. I need to upgrade a few parts though

Most all steer tubes are going to be 1-1/8″ where the stem clamps. You can get most any threadless stem. Short stems are everywhere now. You can go down to basically direct mount stems. 35mm is a pretty standard short stem length. There are two different standards for handlebars: 31.8mm and 35mm so just buy a stem and bars that use the same standard. 31.8 is older and usually cheaper. there are a lot of riser bars these days too. 30 to 40mm rise is a pretty normal “high” rise.

To switch to a 1x drivetrain you can take the front derailleur off and you would need a narrow wide chainring. You may be able to remove the chainrings on your existing bike and fit a narrow wide on the middle slot(a common measurement is 104BCD, you’ll also need nuts and bolts that are the right depth for your crank spider + chainring) but it’s hard to tell. some cheaper cranks have the chainrings riveted on or are just a pain to get off. You’ll need to remove the cranks from the bike either way to get the rings off. if you can’t fit a narrow wide chainring on there you’d need new cranks. 

For the rear derailleur to have enough range you’ll probably need to replace the derailleur, shifter, cassette, and chain with a wide range set. You probably have a wheel with a hyperglide freehub body. You’d mostly be looking for a wide range 10 or 11 speed groupset. Something like a 10 speed deore or a microshift advent or advent x will probably be what you’d want. Higher end 12 speed options are only going to be available on more modern freehub bodies which would probably require a new wheel. There are some budget 12 speed options that will fit an older freehub (it’s generally the cassette that is the limiting factor). One way to tell is often cassettes that fit an older hyperglide freehub body will have 11 teeth on the smallest cog instead of 10 teeth on the nicer ones that require newer standards. 10 and 11 speed drivetrains will be cheaper and easier to adjust/work on yourself. There’s also wide range 8 and 9 speed options these days that are even cheaper but the jump between each gear is bigger making it more difficult to find an optimal cadence which may or may not be an issue depending on your riding.

PNW makes good external dropper posts for reasonable price in a lot of diameters,including the older standards. Best thing to do is to measure your existing sestpost with calipers. Older ones are probably 27.2mm, newer bikes are usually 30.9 or 31.6mm.