Site structure for new business areas

I am a new member and I am about to set up some related wordpress sites for different parts of my business but before I dive in, I desperately need some advice on the best way to structure them.

I have a journal called Art Radar Journal running on Wordpress. I also run an online school called Art Radar Institute. The Institute's courses are held on separate google sites.

I would now like to set up a club for readers and students. This club will eventually include:
? access to the archives of Art Radar
? access to courses which they have paid for
? events
? other services and offerings.

Should I set up the Club

1) on the same site as the journal
2) on a subdirectory of the journal
3) on a new site. or

I do not own so I am using

I am thinking of using art with 3 root directories as follows this will redirect to where the data and software will be stored this will redirect to as above - this will redirect to as above

Or I could do it the other way round and store all the data and plugins on different parts of the art site and have the .com sites redirect into these subdirectories.

Or I could leave the journal on its own as it is well established with a good Google ranking and set up the new areas in art

The issues I am concerned about:

1) google ranking - is it better to put as much content and users onto one site for higher google ranking; should I exploit the existing google ranking of artradarjournal 5/10 by putting the other elements onto that

2) downtime - if I store each area on different .com sites I can use different hosts then if one host goes down, another can still be working

3) security - each area of the business is separate and it will be simpler to restrict users and employees to their own areas. Writers (several) on the journal will not get access to the paid areas (the club and site). If one host's security is compromised, it won't affect the other areas.

4) access - members of the club will need automatic access to areas to journal archives and certain institute courses depending on what they have paid for. I want them to have just one login and password, is this a problem if the data and plugins are on different sites.

5) site address - the names need to be consistent i.e. I don't want one to be and another to and another It has to be organized and self evident.

6) plugin compatibility - using wpmudev will alleviate this problem however wpmudev does not have a publishing plugin which meets our needs at present so we will have to keep the existing one. So there may be compatibility issues. In fact each area needs very different software so there is a big argument for keeping the sites separate to avoid problems with non wpmudev plugin upgrades.

There may be other issues I can't think of.

I would really appreciate some thoughts on this from experienced wordpress users. I have been trying to work out the best way forward for months and it is preventing me from me getting on with developing the separate businesses. I would really like to get it right first time because it is so time-consuming to fix later. I am looking forward to hearing from you.