I would recommend:
1) Dont start with 25 sites, start with 3 or 5, and keep expanding as the time goes by. You might get 25 domains, and start working on the 25th a year or more from now. Unless you are not doing it by yourself.
2) Search and read about creating a network of sites from search engine optimization perspective before you start
3) As far as income I recommend adding popunders from
Traffic Shop, ads from
Juicy Ads or
Exoclick, and selling mobile traffic through a combination of
Plugrush,
Juicy Ads (this one would be an alternate url at plugrush), and
SexGoesMobile.com (this would be an alternate url for
Juicy Ads). I've been using this combination for a while, and the last program does really well, as far as sales.
4) Another tip, dont forget to check seo competition (stay away from high / medium) for the keywords you choose, and put them in domain names. Com, net, org are fine. I would not use co.uk, unless you want to tarket the United Kindgom; this is an extension for a local domain name.
5) As far as hosting, get
Micfo, and get the cheapest plan you possibly can. You can probably go for "Boundless" at 5 - 6 Dollars per month.
6) Technically, it is not legal to use other people content on your website, but I've seen quite a lot of places that don't seem to worry about that. A lot of times people remove the original website logos with Photoshop, or something like that. It would be good to link these images to affiliate links, so you can start generating some sales.
Use Wordpress Multisite and Yoas SEO plugin. You can find instructions on how to set it up on the Internet.
Good luck!