The JCM 800 is a pretty renowned amp, but you'll have to decide what you're going to use it for. 100W amps are great if you're going on tour, but if you're just playing at home they can be a nuisance, b/c you have to crank the volume to get the best tone from it. You can either buy a hot plate/attenuator to cut the volume and preserve the tone, or you can go for a lower wattage amp that has more manageable volume levels.
If you're set on Marshall, they recently released a low wattage tube amp called the Haze. Your local guitar shop should have them - seem pretty decent from the demos I heard online.