Well, as the link from a DIY-projector forum led me here (it is in german, but pictures are multilingual see
www.diy-community.de, or alternatively lumenlab.com, the ultimate resource for that

), I do know a bit about them.
It's basically like an OHP, but just with a "moving slide". Resolution of the projection is the resolution of the LCD used, nad some have built FullHD (1080p) projectors there, nice quality, for ~1k.
But, yeah, basically brightness, resolution and quality is your choice.
Although, while it is a fun project to build one, there are cheaper ways to get hi-quality pics on your wall.
About the lens-in-front-of-your-TV - don't do that. it's barely worth the money. You would have to change the internal settings of your vacuum tube to get a slightly brighter and clearer image, but it is
not really satisfying. OK, that depends on what your expectations are and what you have seen before, but think of it like 5mW red and 50mW green
The ebay projectors for that price are mostly 320*240 resolution - QVGA, even less that NTSC resolution. There, it is far better to get a "real" projector with, e.g. 800*600, with a broken lamp and put a new lamp into it, that might total in ~250$, but with a much better resolution.
But the results are really cool!