I don’t personally have experience with Pacific carriages but I’ve just read their specification and it seems to me that their “Dartmoor” pretty much would do and perform how you’d need it to for single pony cross country phase.
They look pretty well spec’d and manufactured from what I can see. As such I’m entirely confident that it would serve well for single pony.
However is the weight correctly quoted? If so then it’s very light and specified at only 330 lbs is a lot less than would be permitted say for pairs fei or here for any national Driving Trials.
Here and the fei, should be 225 kg for a pair of ponies. That’s about twice the weight they’ve quoted.
It’s o.k. for a single though as that should be minimum of 90 kg or 199 lbs. But you need to be aware that if you have ambition you’d not be able to use it for both singles and pairs of ponies.
Though I do understand that over the water it’s sometimes permitted to drive lower levels in carriages that aren’t permitted higher level - seems mad to me to allow higher risk vehicles at lower level but hey ho… it’s the way of the world! 
You may though want to check out the rules of the competition level you’re thinking of entering.
I personally think though it’s pretty much not really possible to get a carriage that will suit a single and a pair and all phases too without having something you can really adjust to such an extent that you’ve virtually paid as much as buying 2 or 3 carriages.
You should also note that the track width is right on the minimum for the marathon phase. Again that might not bother you but if you’re a novice you might want something that adjusts so you have stability when you need it.
Also again you need to appreciate that for fei it would need to be 138 cms for phase A and C
