The kit instructions suggest treating 32.8 l to achieve the correct amount of wort, but some of those ltrs will get lost in grain absorption, mash tun dead space and kettle losses. You don't need to worry about getting it all the tun as you will only have roughly 15L at a time in there as you will collect your first runnings (I.e. the wort from mashing for 90 minutes before you start to sparge the grains ( by either method)
If you want to test that conversion has happened then the iodine test is probably easiest ( I've never actually used iodine though as I'm confident that if I hit my temps on the mash and leave it for 60-90 minutes conversion happens, also the wort tastes really sweet and not starchy). The extra hydrometer sample that is useful is the pre boil one so that you an split overall efficiency into mash and boil efficiencies later.
The boiler volume is more of an issue, what I used to do was boil any ltr or so surplus in a stock pot on my stove until it had halved in volume. At the end of my boil if I needed to top up my fermenter I used the concentrated wort to top up. It's not ideal but effectively you are making an unhopped liquid malt extract so you aren't diluting your gravity when you need to top up.
What I would recommend is seeing if you can get on one of these
http://www.the-home-brew-shop.co.uk/acatalog/Mashing_Course.html#.UftfnMu9KSM if not watch as much mashing videos on Youtube as possible. There's loads of great stuff out there that makes more sense watching than trying to visualise what I am saying here!
Cheers Tim