That stock sounds a little light IMO, but I'd rather see that than an enormous wish-list of fish! Are you going glass, acrylic or DIY? Going from 2' to 3' width isn't too big a deal, but increasing the height from 2' to 3' will add significantly to the cost, especially with glass as you'd be looking at going from [probably] 12mm to 19mm. I'm sure the fish would love the extra height but it is nowhere as important as the extra width especially as you're not planning on a whole heap of mid-level swimmers.
For the filtration of a tank that size. my preference would be for a large sump. It would be possible to use several large canisters but IMO the cost and difficulty of maintenance doesn't make them a very good option. The only other possibility would be an Ultima-style bead or glass filter, but as I said earlier I'm not sure what is available in the UK. There are many ways you could go about building the sump but the shape and size will depend on how much room you have and what you want to use for filter media. They can be very simple with a couple of filter socks and some bags of noodles/matrix media sitting in there, or as complicated as you like with all sorts of different options for mechanical, biological, chemical and plant filtration. Best thing to do would be to look thru the DIY filters forum and see how others have done things and figure out how best to set it up to suit your application.
ps. what kind of aro are you looking at getting?