How is a new academic journal born?
Here is a first attempt at providing some short suggestions:


You need to identify the scope of the journal and it is best if it does not directly compete with others. Competition is more detrimental if the scope is narrow, it is unavoidable if the scope is wide. You need to make sure it has an international audience (a local journal would probably not end up prestiguous). Find a set of editors that is international in composition, preferably with well known active researchers at the helm. And then you need to look for publishers. I would say that Open Access is what you should go for. Many government funding agencies in Europe now request researchers to publish in Open Access.
Financial in what sense? If you manage to get a commercial publisher to publish your journal they will most certainly set up a budget of some sort. This question is the most difficult to answer. You need to get some finances in to support review systems, type-settings (and by the way, LaTeX would be your way to produce consistent high quality journal papers most easily; you also need someone to design the journal for you). In Open Access journals people usually pay to submit papers, in commercial cases, published papers are charged per page. Many review systems take a fee for each paper submitted. So there will inevitably be some finances involved but they will be determined by the systems you chose.
Yes, without having the journal stamped as serious by established scientists, it will not be easy.
Scientific Society would help but they will probbaly then take over the journal. A publisher/distributor (commercial, society, other body accepted by the community etc.) is a must. You need systems for submitting, managing and publishing manuscripts as well. Getting branded through a publisher is a good way to become recognized.
I would argue that it is only a matter of time before all journals are on-line only. Printing is no longer necessary and really only a waste of time and funding (not to mention resources). Any new journal will almost certainly be on-line only.
A strong line-up of editors and then try to get established scientists to commit to publish in the journal to set a good base-line. You need to work up citations to make the citation index, which takes effort and some time. The citation Index, whether you like it or not, is a hurdle that will make or potentially break any effort. In the long-term people will not published in journals that are not ISI listed.
Chosing the editors should probably be done as publically as possible. I can imagine getting names of persons that are willing to run together and then put them up for a public vote within th ecommunity. This way everyone (who will hopefully send their papers to the j) can feel they are part of the process.
Just make sure it is a communal effort and not a private enterprise. Get as many as possible involved by starting discussions early on in public. Use conferences and listservers (if the community has one) as vehicles for this. Facebook, twitter etc. might also be possible to involve.


EDIT: The journal The Cryosphere is a good and successful  example of a new journal (Open Access). It became ISI listed after a very short time because it received the support of the community in terms of good papers to establish it. It is now a key publication in the field. (and I am not paid to say this!)