I love the little sulky cart! It's very different style from my own, but think, if you wanted that you could still attach a dropped singletree. That would even allow for hames & collar harness if you ever had one of those as well.
On the minis for sale - I've seen them in this state purchased for as low as $50 & as high as $1,000 - from our area and advertised on Craig's List OR at the Dixie Draft Horse Sale (March 2015 was when I went last. Missed Thanksgiving and may not get to go next weekend).
I don't/didn't have a problem with purchasing an older mare for a bit more in 2009, but I bought her specifically to match her full sister for driving and her bloodlines for breeding (even the same height/color/*** and SISTERS didn't matter - they didn't match in stride or personality and it took a long time to get them to work together - but they are one of my pairs now). She was also purchased in foal and with papers current, so again, I had no problem with her price (> $300<$500). We did lose that foal and have had problems since in that department, but I have enjoyed my learning with her AND I have never regretted the purchase. I offered her for sale once, was laughed at by "how much" i wanted for an "old" pony, and have never offered her for sale again. She now drives single, pair, 3 & 4 abreast; rides western or english or is quiet for any lead line child to sit on & pet and is great at community events.
Mare above has gotten attached to her sister - but does drive with a different team mate now. I believe, as she loses her eyesight completely, I would be able to "bell" either her sister or her current partner and have no problems with her following either at home to come in to eat/shelter. Also realize, I did most of my own training and I'm not a pro trainer AND was learning to drive a pair at the same time we were training the above mare(s) as pairs. I DID have lessons with a trainer/instructor every week for 10 months - he was the one that said we'd reached a point where he could no longer help me with that pair. I just needed to get out and drive them. He is not a show trainer, but a draft horse trainer and we worked on basic driving skills - single and pair.