Great questions in regards to the Sarus. I'll go ahead and give you the short answers to each of your questions and refer you to the thread where this subject is being discussed more at length.
Using Temple Guard with the abscense of a Slann has also been discussed recently with various opinions surfacing to support both sides. I'd say that when you are just beginning to play, you probably will find better use for your points than to try and figure out new ways to use a unit that has generally seen a very predefined role (to protect the Slann) at this point. I have provided a link to this thread as well as you may find the discussion usefull in determining wether or not they will be usefull for you at your points level and play level.
http://www.lustria-online.com/threads/are-temple-guard-worthwhile-without-a-slann.1473/
5 front unit gets 20 attacks (5x2)x2
You are absolutely correct. The first two ranks of spear sarus are able to fight. They each have 2x attacks and when fighting with spears so a unit 5 wide that fights in the front when not charging will generate 20 attacks! As a little background, in the previous incantation of the rules (6th Edition), Sarus were only able to fight with 1 attack from the 2nd rank. I think that the reasoning behind this rule, called Predatory Fighters, was to represent the fact that one of the attacks represnted a 'Bite attack' (if you've ever played LM in the Mordheim campaign setting you'll see evidence of this). I think from a rules perspective, simplifying the rules makes the game run smoother and faster for the most part. Now the sarus are just like any other troop armed with spears (given the HE exception.
can i choose whether i use the spears or hand weapons
If you pay the points for the spears, your sarus keep the hand weapons available. You
may choose to use the spears or hand weapons. However, keep in mind that you must make the selection at the beginning of combat and the unit
may not change weapon styles in the middle of combat.
http://www.lustria-online.com/threads/to-spear-or-not-to-spear-that-is-the-question.1656/
If I may offer a comment in regards to adding 2x blocks of 20 Sarus. This seems a little infantry heaving for anything below 2000 points. You'll have a lot of exposed flanks that will need to be protected. Since you'll have a lot less support units (since your points are tied up in sarus blocks), you'll have to play those support units very carefully. I wouldn't advise this play style if you're still learning the game, how your army plays or how the average opponent army plays.
Let us know if you have any other questions and may the dice gods smile upon you.
