1. Not really but you want to make sure it has enough power for you setup.
2. No. Get a charger that can specifically charge Lipos.
3. What ever you were running before should be find. Just watch you motor temps and gear down if it is running too hot.
4. For the most part, yes. I wouldn't submerge them for extended periods of time though.
5.Yes you can which is brings me back to #2 GET A LIPO SPECIFIC CHARGER!!!!!!!
6. Check out the duratrax onyx line of chargers. This would be one of the cheapest its ac/dc and it balance chargers.
[url=http://www.amainhobbies.com/product_info.php/cPath/1574_116/products_id/26108/n/DuraTrax-Onyx-230-AC-DC-Advanced-Charger-w-LCD-Display]DuraTrax Onyx 230 AC/DC Advanced Charger w/LCD Display [DTXP4230] | Electric Accessories - A Main Hobbies[/url]
7.#5 and #2. If money is an issue, checkout hobbypartz.com They have ac/dc chargers but the quality isn't as good as bigger name brand chargers. Also check for used ones on ebay and other forums like rctech, people usually sell them cheaply there. I got a duratrax ice, a 100 dollar charger, for 35 dollars on rctech.net.
For you car, if you get a lipo try to get one with at least 4000mah and 20c but i recommend atleast 5000 mah at 30c. For cheap lipos, check out hobbypartz.com and hobbyking.com just dont get the B-grade lipos.