Thats a mouthfull of questions… I would recommend you to NOT start with multiplayer and/or vehicle game when you are not at least familar with UE…
so, the powerups are quite easy to make: create a basic BP which gets the parent of your 4. Powerups. Add it a collision Box, so you can activate the powerup on overLap events (check collission channels and pickup tutorial videos, there are plenty) and make it dissappear (destroy). The timebased powerups begin the overlap by checking if the overlapped actor is a vehicle (cast) then, apply your effect (modify attributes of the vehicle which passed throuhg the powerup), start a timer which disables the effect on the vehicle. The 5 times I would set a variable on the vehicle of powerupBP-Reference. In your shoot event on the vehicle check if the variable is valid, then retreive its damage increase variable (int) and apply that effect on your weapon or damage affect on the opponent, decrease the variable which holds how often you can use it by 1, if its 0, remove the powerupBP-Reference (set to nothing).
- set the speed of the vehicle to eg 80%
- set it to eg 120%
- in the details of a mesh, you have three relevat options in rendering category: owner no see, only owner see and of course visible. with this settings you can controll that eg. the wheels are “onyl owner see”, the rest of the car is NOT visible.
- create a int variable which defaults to 5. in your powerup, on your overlap set the powerup (self) as a reference to the powerupBP variable, then like I wrote above
On all the steps you need to set the variables serverside and bother with replication, if you really want to do multliplayer