Linky for the above: TRAILER SUSPENSION UNITS 500KG WITH HUBS 4"PCD FULLY GALVANIZED | eBay
Usually sold as pairs & the weight given is the max loading for the pair.
If you intend to use your current wheels check that the replacements have the same PCD as the old: http://www.uk-trailer-parts.co.uk/wo...diameters-pcd/
Also check that the mounting holes in the new ones are the same number & spacing as the old or you'll have to deal with that.
Can I suggest before you buy that you take the trailer fully laden to a local weighbridge & weigh it just to be sure. Can sometimes be a bit of a surprise.
Also what does the plate on your trailer say weightwise?