johm75, Scribus is what you could call an "everything in memory" application. This means that everything in a document - text, images, vectors, etc. - has to be loaded into memory all at once.
If the document - and everything in it - is larger than the available RAM then Scribus will crash. This isn't just an issue with Scribus, most applications are written this way as it's just plain easier to do so. (Swapping stuff in and out of memory at appropriate times is a difficult thing to control.)
Remember that the amount of RAM that you have isn't the amount of RAM available to Scribus. Your OS will take up a sizeable chunk of RAM to begin with and any other application - browser, antivirus, etc. - that is running will also use up memory. (Firefox, for example, can quite happily take up over half a GB if you've done a lot of browsing.)
Also, different OSes have different memory overheads so something that works fine - maybe just coming "under the wire" - on Linux might not work on Windows.
One thing you can do is to restart your computer, then run Scribus without running anything else (obviously keep your security/antivirus stuff running too). Then go to whatever performance/task monitor you have on whichever OS you're using and see how much memory you have spare. This will tell you the absolute maximum size of any file you might be able to use in Scribus. (You can probably only use files quite a bit smaller than this because of other considerations that I'll not go into.)
Then - and this might be tricky unless you've got all of your images in the same folder - find out the size of all of the images you're using in your document. If they're all in the same place then you might just be able to get the size of the folder. Add up the sizes of all of the images and see what that total size is. If this total is larger than the available memory on your system then you can be pretty sure that the document won't load.
If this is the case then, unfortunately, you can't use the document as it is. One stop-gap solution would be to move the images to a different folder which would destroy the links to the images. Then you can open the document and Scribus won't be able to load the images, thus keeping the size of the in-memory document down. Then you can, one by one, check each image and see if it can be cropped or compressed - using something like GIMP - then putting them back into the image frames while checking the available memory. (You can use GIMP to remove unnecessary alpha channels from PNGs too; they take up a lot of space.)
It's not a great solution but it should help you get back on track with a single-document solution.
The other way, as you say, is to create the document in parts/chapters and then stitch the exported PDFs together later. There are lots of applications that will do this; just type "stitch PDF" into a search engine, but beware of the free online services as they might not be nice about what they do with stuff you put through their system (e.g. they might take copyright).
I hope some of this helps.