Good driving Haflingers are worth their weight (and then some) in gold. They can make some fantastic partners and are loads of fun. Ask me how I know!
My boy was trained to drive before I got him, but it took me a while (yeah, three years!) to get around to getting a harness for him. I got an old leather harness given to me secondhand. It was great for the 3 months of ground driving I put into him -- more for my sake than his. And then the breastcollar broke, luckily before I even hitched. Needing another harness pretty quick, and not having a lot of money at the time, I ended up with a Liberty harness from Ron's/Amber Hillside of Canada. It's the same price as the cheap leather and nylon junk you can find on eBay and in the horse.com catalog, but it's really superbly made in comparison. If you're on a budget, you'd be better off with the Liberty harness, even if it is nylon, than the cheap stuff. Amber Hillside Horse Harness - Formerly Rons Horse Harness - Liberty Harness
I've had a good year with it, and the harness is still doing great. I did end up having to get a different breastcollar because the one that came with the harness was way too long for my short bodied golden boy.
But if money's no object, I'd say this is your best bet ... Horse Harness, driving supplies, reins, halters
I'm hoping to upgrade to a Comfy Fit in the next few weeks (or maybe early next month) for my Haflinger. We're ready to go big or go home.
My mare's just about retired from being ridden (may or may not have some back issues that might be made worse by having a rider on her, I've still got to have the vet look at her) but I'm hoping to train her to drive. She'll get the Liberty harness instead, and use it for a good long while and then maybe I'll upgrade her too and keep the nylon set around as a backup.