The 'do not modify' part of the terms of service only applies if you plan on making it public, but since you are only using the pack for your own personal issue than you have no problems.
If you are still uncomfortable using that pack however I wouldn't be able to recommend you another pack but I recommend searching 'mega' in vehicle models and finding a pack that fits your needs.
One thing though, the reason I recommend BlueGhost's pack is because it has almost any vehicle you would ever need in it which makes it perfect for converting replace to addon.
I also personally use the pack for both my replace to addon cars and for the cars it gives, I actually recommend the pack if you want a variety of really well made unmarked vehicles.
Finally one word of advice, if you want to both use a pack for both replace to addon and just for what it adds than copy the main DLC file and rename it to anything you'd like, than in dlclist.xml add the name you wrote for the copied file. After that remove everything relating to the non copied version of the addon DLC you copied in the following places of your copied DLC file vehicle.meta, vehicle.rpf, handling.meta, and carvariations.meta. this is so that the two DLC files don't fight each other, Than edit content.xml and setup2.xml and replace any mention of the non copied addon DLC file name with your DLC file name, finally when you add a car look for either it's closest vanilla GTAV equivalent or it's closest DLC addon equivalent, than copy all of it's data i.e. vehicle.meta, handling.meta etc. into your homemade DLC file's appropriate places, make sure to give the vehicle and it's related data it's own name and your done, than just repeat those last steps for every vehicle you add.
If you follow what I just said just make sure you do one thing, ensure that the dlc.rpf doesn't exceed 5,000,000 KB in size or it can corrupt your game data, if it does get close to exceeding 5,000,000 KB make a second dlc.rpf and name it dlc2.rpf than just move your vehicle files pathway from dlc.rpf to dlc2.rpf and you are good to go.
I hope this helped and I know it is a lot of information so if you have any questions feel free to ask. 👍