As a classic gamer that visits several popular forums like AtariAge, I'd say that such sites are largely full of CRT diehards that will just suggest to you to log onto Craiglist and similar sites and look for cheap late model CRT's like Trinitrons.
Paying some passing attention when such discussion has come up, I think you're best off avoiding the external scaling world. Quality devices are expensive as Darklordjames said and their quality varies greatly from system to system. Something that might be great at upscaling your Sega Genesis might have trouble just even displaying a picture on a Nintendo 64, for instance.
So I suggest taking a different route for now. External upscaler's are just going to get better and more economical for classic gamer's as technology progresses and we move deeper into the HD age when the CRT well starts to dry up. So unless you have deep pockets and are prepared to possibly purchase several expensive devices to satisfy all of your needs, you're better off waiting.
So my advice for you wanting to play this content on your HDTV would be to consider a HTPC. Not only would it be excellent for emulation but I'm sure you would find other handy uses for it as well. Most popular console's before the Dreamcast generation have been emulated to a high degree and even many of the lesser known console's like the Jaguar are making significant progress these day's. And you can even play your original discs with things like Sega CD and Playstation emulators (So no hassle of downloading several hundred MB disc images or ripping your own discs).
I saw a site with a pretty detailed look at many of the scaling options on the marketplace and how they perform with various console's just a few days ago. I'll try to dig it up and post it for you in case it's of any assistance if you decide to go the external scaler route.
Edit - Here's what I saw posted that looks like it has a lot of information on this subject. Hopefully it's of some help in getting an idea of just what your options are in this area.
http://retrogaming.hazard-city.de/Edited by Leo_Ames - 1/23/13 at 11:02pm