Noone should expect you to keep empty boxes for a year. I've changed various gadgets in the past without the box. Some shops may be funny about it, some other shops may say to you that their policy is to refund or exchange products within so many days from purchace. This is rubbish, specific company policies should not affect your statutory rights (I know this to be true in the UK but I'm not sure for Germany). Here in England, usually all you have to say is the two magic words: Trading Standards. If they try to refuse you a refund or exchange and you report them to Trading Standards, they can be fined. But again, I don't know if this is true in Germany.
I'd go for a BenQ DW1650 or DW1655, these two are identical apart from the fact that the 1655 can burn LightScribe.
Some basic tips for trouble-free burning:
If you get an internal drive, make sure you connect your new drive to the secondary IDE port on the motherboard preferably on its own, and set as Master (you have to change the jumper pin at the back of the drive to the Master position).
Use a decent, good quality UDMA4 cable, don't go cheap with things like cables or discs (especially with discs!). Verbatim x16 DVD+R is the media I usually go for as it produces very good results with my BenQ drives. Although it is x16 media, it's best if you burn them at x12, this speed produces the best balance/compromise between quality and burning time.
Make sure you leave your computer alone until burning is finished. Also make sure that your hard drive(s) are defragmented, and that your DVD burner is recognized as UDMA2 in Windows Device Manager (UDMA2 is for BenQ drives, other brands like LiteOn drives operate in UDMA4 mode).
These steps are essential on an older system like yours to ensure decent burns. Once you finish, do some quality scans with Nero CD/DVD Speed and share them with the rest of us. Hope all goes well!