RaZZor1119

Update of SmoothDrivingV is available but only on GTA5-mods.com:
https://www.gta5-mods.com/scripts/smooth-driving-v-2024-update

So, through a combination of different posts and a wonderful mod that pulls everything together, I have managed to implement EUP into my game having never used it before.
 
To summarize for people like me who were confused:
 
If you want to play as your custom character (a new feature in 0.4), and have access to more than just the two models mentioned in the OP, you should probably get EUP. (unless you want to edit the xml files yourself, something I didn't and don't know how to do).
 
If you don't care about the custom character feature, then you can play the same way as you did in the previous version through selecting, as @willpv23 pointed out, the default character option in the switch character menu (which is accessed through the lspdfr tab on the pause screen). This gives you access to the variety of uniforms in the previous version but those uniforms will be worn by the generic, default preset police models in order to do that. If you're good with that (which is how it was in the previous version) than you're good to go. 
 
Of course, you can always edit the xml files yourself of course but people who know how to do that probably aren't in a thread like this.
 
To install EUP:
 
1. Download EUP
Emergency Uniforms Pack - Law and Order by Alex_Ashfold
Emergency Uniforms Pack - Serve and Rescue by Alex_Ashfold (this is extra firemen and EMS skins among others, so its more optional)
Make it easy on yourself and when you download them make sure you have the auto versions as it will give you one file that OpenIV will use and install for you
 
2. Install EUP using OpenIV.
- OpenIV is acquired at OpenIV.com (for first time visitors I know it looks sketchy af but thats how it is lol)
 
- Run install of OpenIV
 
- Upon launch you will have to help the program find your GTAV directory in /steam/steamapps/common/GTAV
 
- Follow instructions for creating the mods folder in your GTAV directory in @FirstThirtyMinutes video here. (I know the video is titled about installing cars, but this portion of the video highlights setting up OpenIV for use which enables EUP installation as well)
 
- After following along with the video you should have a mods folder with the update folder inside
 
- Click edit mode at the top of OpenIV browser (located where filepaths are typically displayed) and you will eventually be prompted to install two things that you must have - ASI loader and OpenIV.ASI - this is done very easily through OpenIV, simply a matter of clicking install when you are prompted. (@FirstThirtyMinutes does this later in the video albeit in a different way). Without them, when you go to install the EUP files, it won't allow you to install in your mods folder that you created which is what you need to do so you don't corrupt the game. 
 
- Get the EUP - LEO 7.5.oiv file and EUP - Serve & Rescue 1.0.oiv (again EUP serve and rescue is more optional so if you opted out you will just have the one file) files from the zip file you download in step 1 and put them on your desktop or somewhere you remember
 
- In OpenIV, click the tools tab, package installer, select the files from your desktop, and install them to your mods folder that you created (OpenIV will give you an option between the mod folder or the GTAV directory, again, its important you have that mod folder and select that as the install location). OpenIV will let you know if you installed it successfully.
 
3. Download and install EUP Outfit Settings
EUP Outfit Settings for 0.4 by mkalash (thanks to @mkalash for his speedy development of this, was surely a time consuming endeavor)
 
Install by dragging the contents of the zip downloaded at the above link into, as per mkalash's instructions on his mod page, into your GTAV/lspdfr/data/custom folder. Simply drag and drop. The custom folder will likely be empty unless you have already messed around with it. 
 
4. Launch LSPDFR normally, select custom character, go to police station, and all the beautiful uniforms from EUP for all different agencies should be available in the locker now if everything was installed correctly.
 
I know this may look daunting at first as a wall of text but I assure you it is much easier than it appears. I would not have been able to figure it out if it wasn't for a combination of FirstThirtyMinutes' older tutorial videos and mkalash's expediently-delivered mod. 
 
These are the steps I took and I hope it can help someone.