Where do you plan on using the beast? Home use, or are you looking for an amp to take out?
If volume is the main concern, the 50watter is not going to be quiet. It is certainly less powerful than the 100w or the Marshall Major 200w plexi, but if you want something you can actually crank in an average size venue, I would be looking for a 2061 or something similar.
I own a 20 watter, and I'm building a couple of my own currently
Having worked at many music stores over the last 6 years I can tell you its a bit tricky finding the right vintage piece. Marshall would probably argue with me on it, but the schematics you find online were NOT always followed. There's a lot of variance in the components used. I've played several late 60s and early 70s amps that should be those "Holy Grail" types and they sounded like shite! haha
Voodoo Amps makes a really good plexi, with really top notch components (Mercury's, Etc) and they're priced very fairly. Each wattage is available either master volume or non-master.
www.voodooamps.comalso check out Reinhardt amps!
Just my 2 cents on the issue