We are using a gallery for images upload with a restriction of image pixel size of 1800 x 1200 and uploading at 80% image quality.

How can our users upload images without resizing and IPB does the image resizing at back end and only upload in specified dimensions: 1800 x 1200 px?

If no existing support exist, im ready to buy/pay for any mods/plugin/custom coding.

If you set Gallery's "large" image size to 1800x1200, when an image is uploaded it will automatically be downsized to these dimensions. The user does not need to do it manually prior to uploading.

To that end, I'm not clear what you are asking for.