I have never put modge podge or anything like that over my finished pages.
You can also use a product called "Gel Medium" It is like a modge podge and comes in a matte, semi-gloss, and glossy finish.
And I always use "gel medium" to adhere things down .. stickers, letters, bit and pieces.... nothing will fall of then.
The one thing I would buy (other than the journal) are black and white marker pens... for doodling and such. And gel medium, gesso, and molding paste.... and ink sprays and acrylic paints ... I know that this is more than one thing.. but these are the things I must have to art journal.
Everyone is different... I know someone who makes stunning art journals using her micron pens and that is all. Hope this helps!