A lot of point you mention there, so think you will get quiet a few varying views.
Gooligum was a good totally free site, but now charges for many of them.
Probably the other good tutorial is by the forums own active assembly guru Nigel.
Plenty of help here to get you going if you cannot get on with tutorials.
The Water sensor , first need to ask if its to be used to gauge the moisture level of soil /potted plants or to detect the actual level of a fluid ?
From what I have read, ( never tried myself) moisture sensors can be very flakey, so you are unlikely to get very accurate long term results.
You might want to look at some of the ready made sensors like this one.
**broken link removed**
Level sensing is easier as there are several ways of doing that.
Those 433mhz modules are very well covered in the forum, assume their range will meet your needs ?
An alternative could be a bluetooth module, range permitting, which means you just plug in a little bluetooth dongle into the pc rather than the need for 2 large boards you need with the 433mhz.
It also has the advantage you can get the Pics data on other devices like your phone.