Selling addons that use workshop models?

I'm not 100% sure.

But I am fairly sure that if you add the owner of the model and ask for permission then you could use it (Obviously also leaving credit in the description of the addon).

But as I said I'm not 100% sure, there may me other rules (EG: TOS) that won't allow this.

I've seen various content creators do this in the past. I'm sure it's fine, however do note it creates a dependency on the workshop author. If that addon was to no longer be available for whatever reason you and your customers are going to be at a loss. Like Graham said it's best to contact that author and work something out with them.

"Even if my addon is literally built of off these models"

You should have made sure of your permission to use them before you did, "it's done anyway so might as well" isn't an excuse

To give a bit of a different perspective:

I think you should opt for a complete set of custom models regardless. Something that can really give an addon the "quality" feel is custom models. When done right models can really make an addon something so much more. Take a look at any of my scripts that involve custom models (mostly by The One Free-Man), without those models I doubt they'd have performed even half as well as they do (Don't worry, Free-man is well compensated for his work). It's not even like you have to drop $100+ on models before you've seen a penny from the script. If you can find the right modeller that's willing to take a gamble on you and make the models for purely a "large" cut of the sales, then you're set. Before you know it you and said modeller will have built not only a pretty good relationship but an easily identifiable brand based on their model style combined with your UI work.

It's a different take on your question, but something to think about none the less.