I want to know; when I pay 10 or 25 euro;
- can I play all the mods given out till now?
- can I play offline on some track against AI?
- for how long am I paying; 10 euro for a week, month, year, forever? (Same with 25 euro of course)
- when I become a junior member at first and get monthly updates, do I miss the weekly updates or do I get them just a bit later; 4 weeks in 1 at the end of the month?
You get access to the monthly build (released on the first Friday of every month) if you pay 10e, or weekly builds (every Friday) if you pay 25e. There aren't any mods, it's all in one build, and you get access to the latest build available at your level. There are people on the forums though that have collected all of the builds (I personally have all weeklies from build 91 onwards and might be willing to share). When stuff is added after the first Friday of the month the 10e members don't get the stuff until they download their next build in the month.
You can play offline by yourself or agains AI, though the AI is very WIP and on many tracks it's mainly useful for crash videos. =)
When you put in your money, you're basically becoming part of the Project CARS team, meaning you'll get access to the forums and the builds according to your level, and when the game is released you'll get your share of the revenue of the game according to how much you bought in for (10e members get a smaller cut than 25e members, who get a smaller cut than the members paying more). 25% will go to SMS, 75% will be payed to those who chipped in. If the game succeeds you can even make a few euros extra if you chip in with 10e now, the break even point is very doable. So 10e will get you to the end and if the game succeeds you might even get it back with some extra.
Every build they release (usually) is a self-sustained copy of the game, they release patches only very rarely, so you'll uninstall and reinstall the game every time you get a new build, meaning that when a new Junior build becomes available to you on the first Friday of the month you'll naturally get everything that was added after the previous monthly build.
EDIT: Hopefully that answers a few questions.