Congrats on the interview, Kari!
That's fabulous.
Meanwhile, back to the suit question. I am going to come at it all in a completely different way.
1. Do you want to work at a place where a suit would be expected for an interview (if not as actual work wear)? If so, that's one good reason to buy a suit!
2. Do you see yourself enjoying or using a suit post-interview (either broken up or as is?) If so, that's another good reason to buy a suit!
3. Or, on the other hand, would wearing a suit feel horrible to you? In that case, perhaps you should not only avoid the suit -- but avoid the job or even the interview for the job!
4. Whether the dress code is conservative or informal -- take note of your feelings as you prepare. Does dressing for the interview feel a little bit fun, or like an interesting challenge? Or does it feel fraught?
I guess what I am saying is, I agree that it is important to dress for the company's culture, and want to add that here, dress is a metaphor.
If you can enter the interview, confident that you know what to wear to make a positive impression, then there's a good chance you'd feel comfortable with the company's culture over the long term. If, on the other hand, you feel completely ill at ease and awkward about what to wear, there's a good chance this is a sign that the place is not a great fit for you.
You don't want to wrestle yourself out of shape to work there -- any more than you want to wear clothes that don't fit or flatter your body.
I also think there are many different kinds of suits. There are conservative and corporate looking suits and there are suits with a lot of fashion flare. (And there are conservative suits worn with flare, which I'm sure you can manage better than anyone!)
Personally, I think a suit would be a great investment. It may or may not be possible to alter it after you have lost weight. But so what? If it's not possible, then you can donate it to DFS and feel great!