So far as I can tell, the AVTs have just recently been discontinued. the last batch were the AVT 'Tributes', the same as the previous production models with different graphics. There are still some new ones in shops but I think that is the end of them. They are still on the Marshall site:
Marshall Amps :: AVT50XT
I haven't played a 50W model but Ive gigged with 100W and 150W ones. They are great for classic rock, not quite up to the all tube designs though.
Here are random opinions from Harmony central:
Marshall VS2000 AVT50 112 Combo: Harmony Central User Reviews
Marshall AVT50X Combo: Harmony Central User Reviews
(dont get too hung up on any one post there, but the overall impression from the reviews of these amps is pretty good)
I don't know about the price, but if it all works and it is in good condition, and you like it, it seems fair IMO. Getting a second hand amp with a known history from someone that you know seems worthwhile to me.
John