You can start work on your project whenever you like, and still use content that isn't unique to your own design. Using free, or purchased content sometimes comes with a terms of use though, so a lot of what you may end up using will simply be placeholders. Using placeholders is perfectly fine until you're capable of producing custom content of your own, or have recruited someone who is willing to do so for you. Since you're already experienced in Unity, and have existing experience in programming you can go as far as creating a playable demo, and can download for free, or purchase online, the content that you need.

In the meantime try and play around with software that has been released, and is available online for free.

As long as your expectations are not sky high, modelling technical objects is not THAT hard.

Go for a non-realistic look, and rather simple textures and shaders, and even some wrong proportions on the models might still be fine. After your first few models, your skill will improve quickly.

One thing to note is: 3D Modelling as such doesn't need an extreme amount of artistic skill as long as you go for modest expecations. It just takes lot of time for a newcomer to finish something.

So rather than asking yourself if it is to hard to do, ask yourself if you can and want to put in the time now to learn 3D modelling... as with everything else, it takes some time to master 3D modelling. And your first few models might not be up to even your most modest expectations, you might want to redo them again before using them for your game.

On the plus side after some time you will be able to create simple models in a timely fashion, and they will look just the way you want them to look. You will still need to invest some time into 3D modelling, but you can save money on the other hand.

Buying the assets on the other hand will give you an instantly usable result. If you have the money, you could also get some rather highend models that would take you as a newcomer ages to finish... if you even manage to do it with your current skilllevel. So if you aim for a realistic graphic, for your skill level in 3D modelling and given that you don't want to spend all your time on learning 3D modelling and creating the models afterwards, buying assets / tracking down free assets might be your only choice... an AAA quality character model takes an incredibel amount of time to create even for an expierienced 3D artist that has various ways to "cheat" (like libraries of pre-made body parts from earlier projects, or clothing templates that only need to be shrink-wrapped and slightly altered for the character, and so on).

Of course, there are downsides beyond just needing to spend money. As already mentioned, assets bought from many different places hardly ever fit together without some tinkering. 3D modelling skills come handy here, so learning the basics of 3D modelling is an essential skill for any Indie working without an artist.

And depending on the amount of assets you need, the cost can add up if byuing assets, or you might have a hard time tracking down a particular free asset.

My suggestion: take some time to at least learn the basics of 3D modelling. It sound daunting at first, but as with programming, once the initial steep learning curve is climbed, you will start to pick up things at a quicker pace.