PC Pro does DVD drive reviews. You can see some here (from July):
http://www.pcpro.co.uk/reviews/87360/lg-gsah10 n.html
There are certain makes that always seem to win these reviews so I would go for one of them, such as NEC, Plextor, Lite-On and LG.
Have a look in W H Smiths (or your library) at the latest PC mags and see if one of them is doing an up to date review.
One thing that may sway you is the software that come with it.
Some offer no software (just the DVD drive) while others may offer Nero, PowerDVD or other software.
If you have not got any DVD burning software you will need it, and if the drive does not come with any you will need to buy it.
So if it comes with the DVD drive it will save you money.
On the subject of PSU, I would doubt that adding a DVD Drive would bother your PSU. Just buy the drive and put it in and I am sure it will work (he says hopefully).
One extra thing, if you have a DVD PLAYER at home, for your TV, make sure that the DVD drive you buy can create DVDs that your DVD player can play (DVD-R or DBD+R)