The larger the sump, the better. If you can fit it, something like a 5 or 6 foot long and 15-18 inch wide tank would be great. Always make sure you have enough room to get your arm over the top and fiddle. Calculate how much you think you will need and work out the height of the tank. Then take another couple of inches off. I have had sumps that barely fit in cabinets before and it is a royal pain.
In a reef that size you will want some large water movement. I would recommend the Tunze Wavebox, it costs a lot (£300), but it simulates the tides coming in and moves the entirety of water in the tank forwards and backwards. You will want to supplement this with some propellor style powerheads (Tunze Streams, Tunze nano streams, hydor koralias are the main ones to use).
For a sump return you will want something that has a max rating of 5-10x your tank's volume so that once head is taken off you still have a fair rate going through. When designing the sump, make sure you have a fair amount given over to the return area (around 1/4 of the length of the sump) to prevent having to constantly top up the system (or install an auto top off).
For a system that size you are really looking at either the top Tunze or (probably a better option) the larger re-circulating Deltec skimmers, something in the AP701/AP851/AP702 sort of range will see you very well (except for the bank balance...).
A durso standpipe, once set up and "tuned" will be very good for you.
Can I be jealous? 
