Firstly no tent is fully waterproof its just some are less waterproof. As to condensation, depending on the difference between the inner and outer temperature, depends on what degree of condensation you will suffer. less so in the summer more so in the winter, and that applies to cheap and expensive tents. In the late spring through to mid Summer you can get away with a single skin tent, winter time having a inner tent helps keep the condensation from dripping onto your sleeping bag, however you will still suffer some, as your breath condensates on the roof of the inner.
If you purchase a lump of a cheap tent the groundsheet are often made of a heavy waterproof material, and so don't suffer being penetrated from stones or sharp objects, where as a expensive backpacking/mountain tent will have a thinner groundsheet which will be waterproof, but more easily penetrated, normally the manufacturers supply a separate groundsheet (Footprint) that you can use if you so wish to put under the tent.
Currently Blacks outdoors are holding a sale of lightweight tents and sleeping bags, personally I would recommend buying a 3 season bag, whilst it might be a tad warm mid summer, by late summer early autumn it will keep you toasty, nothing worse that spending a night in a sleeping bag that not l keeping you warm.
Take a wander around your local outdoor shops, as the staff your questions, hopefully you will get to speak to someone who knows more about the subject than yourself.
As to purifying water, I believe Iodine has now been banned in the EU for water purifying water, Choline tablets are ok but leave taste, personally in the UK I cannot remember using either, I happily drink stream water, just need to ensure there are no dead bodies laying upstream. Anyway I have never had must of a problem just asking at a house if I can fill up my water bottles.
