Hiya! I have a self-published printed comic, by no means an expert but I can give some answers through how it went with me.
Which publishers/printers did you use?
Because I wanted to go self published with my first volume of Petrichor I basically put myself as the publisher and contacted a printer called Pulsio Print that operates from multiple countries in the EU. If you contact them they have some prices that are quite good for small print runs. I was recommended them by a colleague who'd done the same thing as I ended up doing, but a year prior.
I'd say don't hesitate to look around printers and check their specialities and ask for quotes as some of their automatic Price quoting algorithms don't reflect their pricing at low quantities.
When should beginners start printing?
When they have an audience willing to buy a physical copy, a, or preferably multiple outlets to sell the book, AND dry, warm storage space. You MUST have all three before planning on printing books yourself as a self publisher. If you have a publisher or use a print on demand service they'll work on all that, but you have to DIY it if you go alone.
I personnally checked the two latter boxes, having some storage space at home for the amount I was planning on ordering (150 copies) and having an online shop and participating in multiple book fairs and comic cons in artist alley and I'd sold stuff before. The first one I checked with the crowdfunding campaign for the print run that got 52 backers, so I knew that 52 people were willing to put money into the project. I made sure to keep my goals smaller and realistic so the campaign had the highest chance of success and I was blown away by the love and support I got.
How do you know you have enough content to print?
That's on working out your narrative structure. If you're publishing a full story in one book like a standalone novel then the cutoff point is easy: the end. However I'd advise at least one pass of developmental edits (even if that was already done while publishing online, doing another one at the end can't hurt), and a couple of line edit passes before a final spellcheck with beta readers.
If you're publishing a series, make sure each book has it's own arc while the overarching story gets told through the whole series. Whether it's a smaller villain defeated, a character arc touching rock bottom, a cliffhanger where the character just found out they're being cheated on after a book of building romantic bliss or somebody just found the cure to the plague, make sure that the ending of book 1 feels like we've been on a journey and one of the conflicts has found resolution so that the next book can build/bounce off of that.
