The only cosmetic you need is the bombinomicon, if you dont have it then it doesn't really matter

Weapons:
Base jumper
If you die a lot you can use bootlegger/booties
you can use any grenade launcher for some situations too
most recommended launcher is the sticky jumper for movement, because if you use anything else than it would either make you a caberknight or make you just use a normal loadout
and the only melee you will use is the caber, nothing else

using the sticky jumper you can place 2 stickies on the ground then start flying until you reach your closest enemy, if you get lucky with the caber you can actually hit them or more than 1 enemy, if you are using a grenade launcher then it will be easier for you to kill someone.

Again you can use bombinomicon if you want to

For the rocket launcher use the direct hit since it can take out sentries and dispensers easily but if you need splash damage then use stock or a reskin
For your secondary either use mantreads, gunboats, or any banner weapon
You can use the base jumper for soldier if you wanna.
And for your melee either use the equalizer or the escape plan (because of their taunts, but be careful because the taunts kill you too)

if you feel like it, use the equalizer/escape plan taunt on a sniper from behind

Most recommended map for demoman is harvest
For soldier you can do any map
Also you can use tr_walkway_rc2 to practice
Oh yeah and since you are a sentry buster you will mostly kill buildings but mostly sentries
So thats all, bye I guess.