I don't sell harnesses so I am not bias! This is based on 20 years experience of carriage driving, both in the Show Ring, competing in Driving Trials and for pleasure. If you are starting your driving career have a look at the British Driving Society web site.
Your main considerations when buying a harness are SAFETY and COMFORT.
Safety first! As with most things, you get what you pay for. There are cheap leather harnesses available on eBay and elsewhere. These are not as strong as English leather and the leather and stitching is not as strong as English leather or synthetic. If you want an English Leather harness expect to pay 500 pounds plus for a good one, a thousand or more for one by a noted harness maker.
In use, a harness comes under tremendous strain. The full thrust of your horses movement goes through the traces. Snapped traces have led to many serious if not fatal accidents. If you simply must have that multi coloured Morrocan leather harness... for the sake of your horse please get some decent leather or synthetic traces!
There is nothing wrong with a cheap leather harness so long as you inspect it after every use and maintain it properly. Ultimately you might find that what you saved on the price of the harness (as compared to a good quality set of synthetic) you have spent on repairs or replacement parts.
Be wary of the very fancy, very cheap Trade (Gypsy) style harnesses that come up for sale from time to time. They are usualy made of laminated leather... think plywood. As soon as it gets a good drenching it de-laminates
On balance, buying a cheap set of leather harness is a false economy. However, some people just love the look and feel of real leather and take the maintainence and repairs in their stride.
If you are on a budget, something like a set of Libby's would be the thing to get. It is virtually maintainence free and you can pop it in a pillow case and wash it in the washing machine. It drys quickly. This is ideal for winter as you don't have yards of soggy soiled leather to clean.
The unbranded sets of synthetic on eBay usually represent good value for money if you are on a budget or breaking your horse to drive. The breast collars, breeching and saddles often look a bit mean but this is easily remedied by using PollyPads (roller pads) in these areas. These PollyPads are a gret time saver, you simply pop them in the wash and you don't have to clean a whole harness. They distribute the pressure of the back band and collar making even a narrow collar or hard saddle more comfortable.
Another great safety item you can buy on eBay are 'quick releasers' you attach these to the ends of the traces and where the back band attaches to the saddle. You tie bright coloured pieces of cord or rope to the release ring. In the event of an accident you can get your horse out of the cart in seconds and you don't have to cut the harness. These are sold for up to 45 pounds on some driving web sites. Have a look in the Sailing section of eBay and search for "Snap Shackles". You will find exactly he same thing for a fraction of the price. I paid 12 pounds inc. postage for mine.
Names to look out for... Libby's, Tedman. Tedex and Zilco. You can't go wrong with any of these.
When buying English leather... consult the excelent Ebay guide on English Leather Saddlery. This tells you how to tell the difference between English and non English.
When buying an old harness. Ask the seller what it smells like! Ask what the condition the stitching is in, Ask if these are any cracks.
For a 2 wheeler you will need a harness with a sliding back band. Ask the seller if the harness you are interested in has one! Ask if it slides... some are so stiff that they are as good as a fixed back band. If you are going to drive a 4 wheeler with independedent stafts (ie, if one is lifted, the other stays put) you need a harness with a fixed back band. If your 4 wheeler has fixed shafts (like a 2 wheeler) you need a sliding back band.
If you buy a breast collar harness, as opposed to a full collar, you MUST use a swingletree on your carriage or cart. This avoids shoulder injury. You can get away without using a swingle if you are using a full collar.
Some unbranded harnesses are have very strange sizing, Don't be afraid to ask the seller for the measurements!