There are a million stories out there among us photographers. They all deal with bigger and better, and sharper and easier and more elegant, and etc., etc., etc.
What they don't tell you is the fabulous waste of financial resources it took to get to where they are now. Buy high, sell low. Get something for free and then max out your credit card making it work. Getting a bargain on a bigger whatever, only to find that all lenses, paper and darkroom gadgets have to be enlarged as well.
I have been there. I have done that. I have closets full of last year's (decade's/century's) best and most wonderful gear. "If only I had this one more thing, I would be the best in the business!" With that kind of thinking, you will die not so happy and not very rich.
Niko,
You sound like you have a nice camera and useable lenses. Get the 4x5 carrier for your negs and make great images. You don't need bigger negatives than 4x5 to make wonderful images. Look at John Sexton, who shoots 4x5 exclusively. If you absolutely MUST make contact prints, rent or borrow an 8x10. If you love it, save your euros for one, but try before you buy.