I want to insert a few brochure designs I did for a client in InDesign in an ad I'm dsigning in InDesign as well. I'll only need the fronts of the brochures. I'm assuming I should save the two brochures as jpgs then cut out the portions I need in Photoshop, but what resolution should I use?
I would not recommend to use JPG files or any other rasterized format for placing InDesign in InDesign.
I would export an INDD to a PDF/X-4 to retain transparency, and fonts and vectors crispy. And place this PDF/X-4 into InDesign like any other image. This will cause the output with any resolution to have not blurry forms.
Another way could be to place an INDD in INDD as image. This is technically the same as importing a PDF but you have also to take care that all links and fonts are available. If only one page is used I think the first step has less amount of data.
Place the live InDesign brochure files into you new InDesign document
or - place pdf files of the brochure into InDesign
InDesign jpg export is not as good as Photoshop jpg, btw