Whatever you get, I would recommend a dual frequency L1/L2 receiver. The next few years are going to be highly solar active and the dual frequency will be a lot more accurate when correcting for errors induced by the ionosphere. With a single frequency antenna they rely on A predetermined model, whereas the dual figures the error on it's own.
You will also get cm accuracy with the right subscription to a corrections provider, but it will cost you, we are paying just shy of 100k per year for our subscriptions, however that's probably overkill for you.
Also, make sure the gps is GLONASS ready as you will get almost double the satellites and of course the more the merrier.