You don't need 1080p, you will find that most LCDs at this size are though, although possibly not in your price range. I would say that the 50PQ3000 is the best set you will find for the money though. With plasma you will get richer and more natural colours, better motion handling, better SD picture (although that shouldn't be an issue as the V+ box will upscale everything) and deeper black levels. LCDs are catching up in all of these areas, especially blacks and motion, but I haven't seen an LCD which matches an eqiuvalently priced plasma yet.
I would doubt that you will have an issue with screen burn, unless you spend hours watching the news channels (or anyother channels) with very bright logos which remain on screen, things like the red button can be removed so bare that in mind. You may get some image retention, particularly after watching a movie the black bars will probably appear to faintly stay there, but this will dissapear in seconds. Bright channel logos like the ITV 2, 3 and 4 logos may also be retained, but most logos dissapear during ad breaks, this gives the TV enough time to wash away at least most of any image retention so it never gets too bad.
To be safe it would be a good idea to lower the contrast and colour settings in the first 200 hours or so as this is when it's most prone to the problem. Alternatively you could see if there is a Panasonic model going for a similar price as Panasonics are more resistant to the problem, the P50C10 would be the one to look at, although I'm unsure whether it'll match the LG in picture quality, I'm sure it will be a close call though.
EDIT: you may want to post in the plasma forums for more information on burn in (or do a search on there) or for more information on the specific plasma models you're looking at