Hey dan! Welcome to the board.
I know it's now monday, so you will have allready been to see the bike. I may be a bit late, but I just seen your post a minute ago, and thought that I might as well reply.
I bought my B4 for £850, and I feel like I got such a good deal on it. It was excellent condition, brand new tyres, well looked after. Sunds like the one you were looking at is good condition too. If it looks as good as it sounds, like mine did, I would say go for it. I'm well happy with my little Bandit.
You were asking how it would compare to other 400s.... Well, I've never actually even seen another 400 where I live, but my mate has a Triumph Daytona 600, and they seem to accelerate pretty equally up until about 100mph. And If I give it some go, he seems to have a hard time keeping up! Also, I managed to keep up with him, and a couple of guys on gixxers and fireblades on the motorway, but a fazer600 who was travelling with us couldn't keep up. The B4 is indeed quite a nippy little bike.
You did say the one you are looking at is restricted though... While that shouldn't affect the acceleration, you won't be able to get the same top speed as you would without the restrictor kit. My unrestriced bike can get about 128mph, as indicated on the speedo. I'm not sure what a restricted bandit will do, but I've heard it will still do 100mph, and I even heard of one guy who could do 120mph with his restricted B4. Still, you could take the restrictor kit out. I've heard it's easy enough (a couple of washers in the carbs that you need to remove.) If you have a full unrestricted licence you can do this legally, but if you have the restricted (A2) license, you legally should keep the restrictor in. But the fuzz can't easily check if it's there or not, so as long as you have a certificate to say it's restricted to 33bhp, then that should keep them happy.
I notice you said £850
ono... Not that the bike isn't worth £850, but if the seller is open to lower offers, you might as well try and get it down a bit. Offer £800, see what he says!!
Hope some of that is relevant and helps you out. Let us know how things go.