hi doratheexplorer! the air + land package (includes all breakfast and 4 dinners, airport transfers, sightseeing tours, and porterage) cost a little more than USD 3000. Optional tours, if you plan to take them all, will set you back a little less than USD1000 more.
krishakulit: okay, let me orgainize my thoughts

-regarding souvenirs... i collect decorative plates from each country that I go to. they look really nice as a group. price ranges from 4 euros to 30 euros (depending on the design and size).
food souvenirs are always cheap and always appreciated. chocalates from switzerland, cheese and wine from italy, champagne from france, tea from UK, etc. Again, hit the supermarkets for these!
Venetian murano glass (from jewelries to glass sets) and masks, perfume, and clothes stores like nuna lie and promod (cheap), foie gras, perfume (sephora is a one-stop shop) dijon mustard, etc., florentine leather (gold embossed leather belts, shoes, bags, trash bin, you name it, Peruzzi seem to be the largest store), casagrande in lucerne got the largest selection of souvenirs such as cow bells, cookoo clocks, swiss army knives, etc., then in london, hit oxford st. and kensington high st. for brands like top shop (huge store, suggest to buy boots from here, payed USD90 for mine but it was leather and design was unique), h&m, uniqlo, etc., paddington bear, any Underground (tube) souvenirs are always cool, Harrod's bag, Boots cosmetics, etc.
-regarding water, refrain from drinking! but seriously, you can't drink a lot because coach rides are long and rest stops are minimum 2 hrs apart. suggest you buy water in the supermarket, way cheaper!
-travel journal is always a good thing, though it's all in my memory! yes, it was that memorable! my pictures helped too... scrap booking is a good idea, old school

try shutterfly photo booking, same concept.
-I would think it'll be much warmer on April...but then again, I haven't gone to Europe this time of the year. suggest to always bring comfortable shoes (no heels but stylish!), good size bag, and plenty of clothes!
check fashion spot (streetstyle) to see the latest fashion in europe, always helps to blend in with the locals... I found europeans to be much more stylish than americans and they like wearing a lot of leather jacket and shoes, leggings/tights, clothes/coats with good fit, oversized bags (not necessarily designer), flats, vintage, etc. dunno about spring though...
laundromats are hard to find in europe, suggest to bring tons of tops and intimates! recycle your bottoms, coats, and shoes (can't do much about this). found the sink wash-blow dry technique to be messy and impossible. in terms of packing, always leave space for souvenirs! just bring what you will use and put all your valuables in your carry on/personal bag. use a money belt for cash, passport, and credit cards.
-24 days for touring is looong! oh my... it might be a good choice to join a tour that covers all meals then. figure about 5 euros for train rides, 15-20 euros for short taxi rides (now london is a different story, take the tube instead), snacks like gelato (about 3 euros) and panini/sandwhiches (4-5 euros), coffee (1-2 euros), water/softdrinks(3-5 euros), toilette (at least 25 cents), etc.
-can't help you much with cell phone matters... i just left the hotels' telephone numbers with my family so they can reach me. didn't plan to spoil my vacation with phone calls anyway.
-internet is expensive at the hotels, so ask the locals where the internet cafes are.
-regading money matters... for large purchases, use plastic and for everyday expenses, get money from the wall! btw, do let your bank know you'll be using your atm card/cc in so and so countries, or else, you might not be able to use 'em.
never exchange money at home before going to europe, bad idea! exchange rate is the worste! worse is changing money on banks/hotels, plus they get commission. credit cards are better with exchange rate but charges conversion and transaction? fees (still not bad,depends on type of cc). money from atm offer the same exchange rate as credit card(best) and only charges minimal withdrawal and conversion fees.
I know it's confusing but ask any expert traveller and even your tour director, he/she'll suggest you use the atm. you can use any atm machine you see. only thing is that some of them like in my case, don't charge any withdrawal fees. case in point, since my bank (bank of america) is affiliated with BNP paribas ? in paris, I wasn't charged any fees.
Whew! that was a mouthful... if you think of other q's, i'll be around. cheers!