1.) USB's transfer rate is 480mbits/s or 60mb/s. I don't think you can expect that much though, after doing some research it seemed like you can expect maybe 25~mb/s. I think it depends on the external harddrive though.
2.) If you configure it to save them onto the drive instead of your main one.)
3.)Yes.
4.)Yes.
5.)Yes, but there may be some problems, since you won't be bringing the registry files that programs install along with you on your drive.
6.)Depends on the normal enclosure. You should be fine, though, since both enclosures and external harddrives are designed with cooling in mind. Some enclosures run cooler than others.
7.)Depends on the actual harddrive inside, if there is any fans in it, what the enclosure is made out of. Also depends on what interal harddrive setup you're comparing it too.
Personally, I would get a 750GB HDD and put it in my own enclosure. 750GB HDD's are a lot more common, and have a much better price at this point. I do not think you'd find a good HDD + enclosure deal for under 250$
My recommendations:
HDD:
http://www.newegg.com/Product/Product.aspx?Item=N82E16822152100Enclosure :
http://www.newegg.com/Product/Product.aspx?Item=N82E16817145126
None.
Assembly tricky? No, just requires screwing a few screws (if that) and plugging in some cables.
Just looking on NewEgg for something offering something similar, I came across this
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136176It's a bit bulkier, I think. It is one of the cheaper 750gb external HDDs from a brand that I trust.
Simpler? Both are simple processes, I think. There's probably more work invloved in installing an internal HDD rather than an external HDD.
None.