Well for more tips than I could ever give you on useing Movie Maker 2 take a look at
www.papajohn.org
As for an external DVD writer then I have just had a quick look on
www.ebuyer.co.uk and found the following that looks good:
Liteon 1633SX 16x DVD R/RW Dual Layer External USB 2.0
Quickfind code: 76821
£77.54 inc VAT
or another option would be to buy your own external case and put the DVD writer of your choice in it. This can often work out cheaper and you get exactly what you want. I would always recommend the Pioneer DVD writers, the latest one being the DVR-108 drive for around £57 plus about £30 for the case.
Mark.