hey mate, do a search in the forums, bound to find a load of stuff that is helpful.
As for the questions:
1. i prefer tank and van. alot start on back pack etc but most generally move to tank and van setup. I used to have filters in van but now have static setup in brother in laws shed. Depends on whether your water metered/water pressure/drive access etc.
2. Not too hard, if you know what you are doing. I am rubbish at DIY but with a litle help could work things out. Helps if you know someone who can help you. Or have the internet!
3. Dont know bout this, i would buy new filter system as dont know where its been otherwise! I used june at gaps water for mine, very helpful, excellent after sales too.
But seriously, do a search thru forums, theres a wealth of info on here regarding what to look out for setup wise your situation and location dependant.