Hard to maintain compared to what exactly? Compard to a Jeep? Then it's a nightmare, compared to other armor . . . not too bad at all. The British plug and weld the barrel but that will NOT pass muster by the ATF. As I understand it now you won't get the barrel into the US at all these days. I had to cut the breach and we cut a bore sized slot in the barrel but that was 10 years ago.
I've been 65 mph and it was still accelerating and it is a VERY smooth ride.
You can get any parts you want but it'll cost you, PLUS shipping from the UK. Based on the last set of tracks I bought, plus shipping, it costs me $12-15 per mile in track wear alone. Aside from grass, I don't use it off road.
As for size, I'm 6'2" and was still able to get into it and use it when I weighed 350+ lb. OK, it's much easier now that I weight 225.