The EVH 5150III LBX series is around 20 watts, and can be picked up relatively cheap if used.
I have the 'original' LBX, and IMO it will not do Marshall or classic rock tones. The red channel is basically the 'burn' channel of the full-sized 5150, and the blue channel simply removes two of the red channel's gain stages, making it have something like the high gain preamp in a SLO or Mesa Recto. It just amounts to too much gain for the classic rock stuff, even with the gain turned way down on the blue channel.
Having said that, the actual blue channel on the full-sized 5150 does approximate more of the Marshall sound and gain levels.
And the LBX II model specifically does have the blue channel that the full-sized 5150 does.
Of course this prompts the question of just how Marshall-like, and which Marshall does the 5150 blue channel sound like? That really ends up being subjectively answered, but I think it sounds at least somewhat like a 2203/2204 JCM800.
You can also typically find 5150III heads for sometimes very cheap used (sometimes for not much more than the LBX heads), but at 50 watts, those suckers are seriously loud. Yes, this amp has a MV, but IME you can only go so low before things no longer sound very good.
Anyway, the 5150 line of amps is really affordable, especially used. Whether or not they do Marshall tones to anyone's subjective tastes is going to be a bit of a gamble.